a2p
accept
access
acct
addftinfo
addr2line
adjtime
afmtodit
after
aio_cancel
aio_error
aio_read
aio_return
aio_suspend
aio_waitcomplete
aio_write
alias
aliases
alloc
anvil
append
apply
apropos
ar
array
as
asa
asn1parse
at
atq
atrm
attemptckalloc
attemptckrealloc
authlib
authtest
autopoint
awk
b64decode
b64encode
basename
batch
bc
bdes
bell
bg
bgerror
biff
big5
binary
bind
bindkey
bindtags
bindtextdomain
bio
bitmap
blowfish
bn
bootparams
bootptab
bounce
brandelf
break
breaksw
brk
bsdiff
bsdtar
bsnmpd
bspatch
bthost
btsockstat
buffer
builtin
builtins
bunzip2
button
byacc
bzcat
bzegrep
bzfgrep
bzgrep
bzip2
c2ph
c89
c99
ca
cal
calendar
canvas
cap_mkdb
case
cat
catch
catman
cc
cd
cdcontrol
chdir
checkbutton
checknr
chflags
chfn
chgrp
chio
chkey
chmod
chown
chpass
chroot
chsh
ci
ciphers
ckalloc
ckdist
ckfree
ckrealloc
cksum
cleanup
clear
clipboard
clock
clock_getres
clock_gettime
clock_settime
close
cmp
co
col
colcrt
colldef
colors
colrm
column
comm
command
compile_et
complete
compress
concat
config
connect
console
continue
core
courierlogger
couriertcpd
cp
cpan
cpio
cpp
creat
crl
crontab
crunchgen
crunchide
crypt
crypto
csh
csplit
ctags
ctm
ctm_dequeue
ctm_rmail
ctm_smail
cu
cursor
cursors
cut
cvs
date
dbiprof
dbiproxy
dc
dcgettext
dcngettext
dd
dde
default
defer
deliverquota
des
destroy
devfs
df
dgettext
dgst
dh
dhparam
dialog
diff
diff3
dig
dir
dirent
dirname
dirs
discard
disktab
dngettext
do
domainname
done
dprofpp
dsa
dsaparam
dtmfdecode
du
dup
dup2
eaccess
ec
ecdsa
echo
echotc
ecparam
ed
edit
editrc
ee
egrep
elf
elfdump
elif
else
enc
enc2xs
encoding
end
endif
endsw
engine
enigma
entry
env
envsubst
eof
eqn
err
errno
error
errstr
esac
ethers
euc
eui64
eval
event
evp
ex
exec
execve
exit
expand
export
exports
expr
extattr
extattr_delete_fd
extattr_delete_file
extattr_get_fd
extattr_get_file
extattr_set_fd
extattr_set_file
f77
false
famm
famx
fblocked
fbtab
fc
fchdir
fchflags
fchmod
fchown
fcntl
fconfigure
fcopy
fdescfs
fdformat
fdread
fdwrite
fetch
fg
fgrep
fhopen
fhstat
fhstatfs
fi
file
file2c
fileevent
filename
filetest
find
find2perl
finger
flex
flock
flush
fmt
focus
fold
font
fontedit
for
foreach
fork
format
forward
fpathconf
frame
from
fs
fstab
fstat
fstatfs
fsync
ftp
ftpchroot
ftpusers
ftruncate
futimes
g711conv
gb2312
gb18030
gbk
gcc
gcore
gcov
gdb
gencat
gendsa
genrsa
gensnmptree
getconf
getdents
getdirentries
getdtablesize
getegid
geteuid
getfacl
getfh
getfsstat
getgid
getgroups
getitimer
getlogin
getopt
getopts
getpeername
getpgid
getpgrp
getpid
getppid
getpriority
getresgid
getresuid
getrlimit
getrusage
gets
getsid
getsockname
getsockopt
gettext
gettextize
gettimeofday
gettytab
getuid
glob
global
gmake
goto
gperf
gprof
grab
grep
grid
grn
grodvi
groff
groff_font
groff_out
groff_tmac
grog
grolbp
grolj4
grops
grotty
group
groups
gunzip
gzcat
gzexe
gzip
h2ph
h2xs
hash
hashstat
hd
head
help2man
hesinfo
hexdump
history
host
hostname
hosts
hosts_access
hosts_options
hpftodit
http
hup
i386_get_ioperm
i386_get_ldt
i386_set_ioperm
i386_set_ldt
i386_vm86
iconv
id
ident
idprio
if
ifnames253
ifnames259
image
imapd
incr
indent
indxbib
info
infokey
inode
install
instmodsh
interp
intro
introduction
ioctl
ipcrm
ipcs
ipf
ipftest
ipnat
ippool
ipresend
issetugid
jail
jail_attach
jobid
jobs
join
jot
kbdcontrol
kbdmap
kcon
kdestroy
kdump
kenv
kevent
keycap
keylogin
keylogout
keymap
keysyms
kgdb
kill
killall
killpg
kinit
kldfind
kldfirstmod
kldload
kldnext
kldstat
kldsym
kldunload
klist
kpasswd
kqueue
kse
kse_create
kse_exit
kse_release
kse_switchin
kse_thr_interrupt
kse_wakeup
ktrace
label
labelframe
lam
lappend
last
lastcomm
lastlog
lchflags
lchmod
lchown
ld
ldap
ldapadd
ldapcompare
ldapdelete
ldapmodify
ldapmodrdn
ldappasswd
ldapsearch
ldapwhoami
ldd
leave
less
lesskey
lex
lgetfh
lhash
libnetcfg
library
limit
limits
lindex
link
linprocfs
linsert
lint
lio_listio
list
listbox
listen
lj4_font
lkbib
llength
lmtp
ln
load
loadfont
local
locale
locate
lock
lockf
log
logger
login
logins
logname
logout
look
lookbib
lorder
lower
lp
lpq
lpr
lprm
lptest
lrange
lreplace
ls
lsearch
lseek
lset
lsort
lstat
lsvfs
lutimes
lynx
m4
madvise
magic
mail
maildiracl
maildirkw
maildirmake
mailq
mailx
make
makeinfo
makewhatis
man
manpath
master
mc
mcedit
mcview
md2
md4
md5
mdc2
memory
menu
menubar
menubutton
merge
mesg
message
mincore
minherit
minigzip
mkdep
mkdir
mkfifo
mkimapdcert
mklocale
mknod
mkpop3dcert
mkstr
mktemp
mlock
mlockall
mmap
mmroff
modfind
modfnext
modnext
modstat
moduli
more
motd
mount
mprotect
mptable
msdos
msdosfs
msgattrib
msgcat
msgcmp
msgcomm
msgconv
msgen
msgexec
msgfilter
msgfmt
msggrep
msginit
msgmerge
msgs
msgunfmt
msguniq
mskanji
msql2mysql
msync
mt
munlock
munlockall
munmap
mv
myisamchk
myisamlog
myisampack
mysql
mysqlaccess
mysqladmin
mysqlbinlog
mysqlcheck
mysqld
mysqldump
mysqld_multi
mysqld_safe
mysqlhotcopy
mysqlimport
mysqlshow
mysql_config
mysql_fix_privilege_tables
mysql_zap
namespace
nanosleep
nawk
nc
ncal
ncplist
ncplogin
ncplogout
neqn
netconfig
netgroup
netid
netstat
networks
newaliases
newgrp
nex
nfsstat
nfssvc
ngettext
nice
nl
nm
nmount
nohup
nologin
notify
nroff
nseq
nslookup
ntp_adjtime
ntp_gettime
nvi
nview
objcopy
objdump
objformat
ocsp
od
onintr
open
openssl
opieaccess
opieinfo
opiekey
opiekeys
opiepasswd
option
options
oqmgr
pack
package
packagens
pagesize
palette
pam_auth
panedwindow
parray
passwd
paste
patch
pathchk
pathconf
pawd
pax
pbm
pcre
pcreapi
pcrebuild
pcrecallout
pcrecompat
pcrecpp
pcregrep
pcrematching
pcrepartial
pcrepattern
pcreperform
pcreposix
pcreprecompile
pcresample
pcretest
perl
perl56delta
perl58delta
perl561delta
perl570delta
perl571delta
perl572delta
perl573delta
perl581delta
perl582delta
perl583delta
perl584delta
perl585delta
perl586delta
perl587delta
perl588delta
perl5004delta
perl5005delta
perlaix
perlamiga
perlapi
perlapio
perlapollo
perlartistic
perlbeos
perlbook
perlboot
perlbot
perlbs2000
perlbug
perlcall
perlcc
perlce
perlcheat
perlclib
perlcn
perlcompile
perlcygwin
perldata
perldbmfilter
perldebguts
perldebtut
perldebug
perldelta
perldgux
perldiag
perldoc
perldos
perldsc
perlebcdic
perlembed
perlepoc
perlfaq
perlfaq1
perlfaq2
perlfaq3
perlfaq4
perlfaq5
perlfaq6
perlfaq7
perlfaq8
perlfaq9
perlfilter
perlfork
perlform
perlfreebsd
perlfunc
perlglossary
perlgpl
perlguts
perlhack
perlhist
perlhpux
perlhurd
perlintern
perlintro
perliol
perlipc
perlirix
perlivp
perljp
perlko
perllexwarn
perllinux
perllocale
perllol
perlmachten
perlmacos
perlmacosx
perlmint
perlmod
perlmodinstall
perlmodlib
perlmodstyle
perlmpeix
perlnetware
perlnewmod
perlnumber
perlobj
perlop
perlopenbsd
perlopentut
perlos2
perlos390
perlos400
perlothrtut
perlpacktut
perlplan9
perlpod
perlpodspec
perlport
perlqnx
perlre
perlref
perlreftut
perlrequick
perlreref
perlretut
perlrun
perlsec
perlsolaris
perlstyle
perlsub
perlsyn
perlthrtut
perltie
perltoc
perltodo
perltooc
perltoot
perltrap
perltru64
perltw
perlunicode
perluniintro
perlutil
perluts
perlvar
perlvmesa
perlvms
perlvos
perlwin32
perlxs
perlxstut
perror
pfbtops
pftp
pgrep
phones
photo
pic
pickup
piconv
pid
pipe
pkcs7
pkcs8
pkcs12
pkg_add
pkg_check
pkg_create
pkg_delete
pkg_info
pkg_sign
pkg_version
pkill
pl2pm
place
pod2html
pod2latex
pod2man
pod2text
pod2usage
podchecker
podselect
poll
popd
popup
posix_madvise
postalias
postcat
postconf
postdrop
postfix
postkick
postlock
postlog
postmap
postqueue
postsuper
pr
pread
preadv
printcap
printenv
printf
proc
procfs
profil
protocols
prove
proxymap
ps
psed
psroff
pstruct
ptrace
publickey
pushd
puts
pwd
pwrite
pwritev
qmgr
qmqpd
quota
quotactl
radiobutton
raise
rand
ranlib
rcp
rcs
rcsclean
rcsdiff
rcsfile
rcsfreeze
rcsintro
rcsmerge
read
readelf
readlink
readonly
readv
realpath
reboot
recv
recvfrom
recvmsg
red
ree
refer
regexp
registry
regsub
rehash
remote
rename
repeat
replace
req
reset
resolver
resource
return
rev
revoke
rfcomm_sppd
rfork
rhosts
ripemd
ripemd160
rlog
rlogin
rm
rmd160
rmdir
rpc
rpcgen
rs
rsa
rsautl
rsh
rtld
rtprio
rup
ruptime
rusers
rwall
rwho
s2p
safe
sasl
sasldblistusers2
saslpasswd2
sbrk
scache
scale
scan
sched
sched_getparam
sched_getscheduler
sched_get_priority_max
sched_get_priority_min
sched_rr_get_interval
sched_setparam
sched_setscheduler
sched_yield
scon
scp
script
scrollbar
sdiff
sed
seek
select
selection
semctl
semget
semop
send
sendbug
sendfile
sendmail
sendmsg
sendto
services
sess_id
set
setegid
setenv
seteuid
setfacl
setgid
setgroups
setitimer
setlogin
setpgid
setpgrp
setpriority
setregid
setresgid
setresuid
setreuid
setrlimit
setsid
setsockopt
settc
settimeofday
setty
setuid
setvar
sftp
sh
sha
sha1
sha256
shar
shells
shift
shmat
shmctl
shmdt
shmget
showq
shutdown
sigaction
sigaltstack
sigblock
sigmask
sigpause
sigpending
sigprocmask
sigreturn
sigsetmask
sigstack
sigsuspend
sigvec
sigwait
size
slapadd
slapcat
slapd
slapdn
slapindex
slappasswd
slaptest
sleep
slogin
slurpd
smbutil
smime
smtp
smtpd
socket
socketpair
sockstat
soelim
sort
source
spawn
speed
spinbox
spkac
splain
split
squid
squid_ldap_auth
squid_ldap_group
squid_unix_group
sscop
ssh
sshd_config
ssh_config
stab
startslip
stat
statfs
stop
string
strings
strip
stty
su
subst
sum
suspend
swapoff
swapon
switch
symlink
sync
sysarch
syscall
sysconftool
sysconftoolcheck
systat
s_client
s_server
s_time
tabs
tail
talk
tar
tbl
tclsh
tcltest
tclvars
tcopy
tcpdump
tcpslice
tcsh
tee
tell
telltc
telnet
term
termcap
terminfo
test
texindex
texinfo
text
textdomain
tfmtodit
tftp
then
threads
time
tip
tk
tkerror
tkvars
tkwait
tlsmgr
tmac
top
toplevel
touch
tput
tr
trace
trafshow
trap
troff
true
truncate
truss
tset
tsort
tty
ttys
type
tzfile
ui
ul
ulimit
umask
unalias
uname
uncomplete
uncompress
undelete
unexpand
unhash
unifdef
unifdefall
uniq
units
unknown
unlimit
unlink
unmount
unset
unsetenv
until
unvis
update
uplevel
uptime
upvar
usbhidaction
usbhidctl
users
utf8
utimes
utmp
utrace
uudecode
uuencode
uuidgen
vacation
variable
verify
version
vfork
vgrind
vgrindefs
vi
vidcontrol
vidfont
view
virtual
vis
vt220keys
vwait
w
wait
wait3
wait4
waitpid
wall
wc
wget
what
whatis
where
whereis
which
while
who
whoami
whois
window
winfo
wish
wm
write
writev
wtmp
x509
xargs
xgettext
xmlwf
xstr
xsubpp
yacc
yes
ypcat
ypchfn
ypchpass
ypchsh
ypmatch
yppasswd
ypwhich
yyfix
zcat
zcmp
zdiff
zegrep
zfgrep
zforce
zgrep
zmore
znew
_exit
__syscall
operator is not equiva- lent to the tr(1) utility. If you want to map strings between lower/upper cases, see "lc" in perlfunc and "uc" in perlfunc, and in general consider using the "s" operator if you need reg- ular expressions. Note also that the whole range idea is rather unportable between character sets--and even within character sets they may cause results you probably didn't expect. A sound principle is to use only ranges that begin from and end at either alphabets of equal case (a-e, A-E), or digits (0-4). Anything else is unsafe. If in doubt, spell out the character sets in full. Options: c Complement the SEARCHLIST. d Delete found but unreplaced characters. s Squash duplicate replaced characters. If the "/c" modifier is specified, the SEARCHLIST character set is complemented. If the "/d" modifier is specified, any char- acters specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some tr programs, which delete anything they find in the SEARCHLIST, period.) If the "/s" modifier is specified, sequences of characters that were transliterated to the same character are squashed down to a single instance of the charac- ter. If the "/d" modifier is used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACE- MENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class. Examples: $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case $cnt = tr/*/*/; # count the stars in $_ $cnt = $sky =~ tr/*/*/; # count the stars in $sky $cnt = tr/0-9//; # count the digits in $_ tr/a-zA-Z//s; # bookkeeper -> bokeper ($HOST = $host) =~ tr/a-z/A-Z/; tr/a-zA-Z/ /cs; # change non-alphas to single space tr [\200-\377] [\000-\177]; # delete 8th bit If multiple transliterations are given for a character, only the first one is used: tr/AAA/XYZ/ will transliterate any A to X. Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval(): eval "tr/$oldlist/$newlist/"; die $@ if $@; eval "tr/$oldlist/$newlist/, 1" or die $@; <Can't find string terminator "END" anywhere before EOF.... Additionally, the quoting rules for the identifier are not related to Perl's quoting rules -- "q()", "qq()", and the like are not supported in place of '' and "", and the only interpo- lation is for backslashing the quoting character: print << "abc\"def"; testing... abc"def Finally, quoted strings cannot span multiple lines. The gen- eral rule is that the identifier must be a string literal. Stick with that, and you should be safe. Gory details of parsing quoted constructs When presented with something that might have several different inter- pretations, Perl uses the DWIM (that's "Do What I Mean") principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant. This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together. The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. The other rules are likely to contradict the user's expectations much less fre- quently than this first one. Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For differ- ent quoting constructs, Perl performs different numbers of passes, from one to five, but these passes are always performed in the same order. Finding the end The first pass is finding the end of the quoted construct, whether it be a multicharacter delimiter "\nEOF\n" in the "<" which terminates a fileglob started with "<". When searching for single-character non-pairing delimiters, such as "/", combinations of "\\" and "\/" are skipped. However, when searching for single-character pairing delimiter like "[", combina- tions of "\\", "\]", and "\[" are all skipped, and nested "[", "]" are skipped as well. When searching for multicharacter delimiters, nothing is skipped. For constructs with three-part delimiters ("s///", "y///", and "tr///"), the search is repeated once more. During this search no attention is paid to the semantics of the construct. Thus: "$hash{"$foo/$bar"}" or: m/ bar # NOT a comment, this slash / terminated m//! /x do not form legal quoted expressions. The quoted part ends on the first """ and "/", and the rest happens to be a syntax error. Because the slash that terminated "m//" was followed by a "SPACE", the example above is not "m//x", but rather "m//" with no "/x" mod- ifier. So the embedded "#" is interpreted as a literal "#". Also no attention is paid to "\c\" during this search. Thus the second "\" in "qq/\c\/" is interpreted as a part of "\/", and the following "/" is not recognized as a delimiter. Instead, use "\034" or "\x1c" at the end of quoted constructs. Removal of backslashes before delimiters During the second pass, text between the starting and ending delim- iters is copied to a safe location, and the "\" is removed from combinations consisting of "\" and delimiter--or delimiters, mean- ing both starting and ending delimiters will should these differ. This removal does not happen for multi-character delimiters. Note that the combination "\\" is left intact, just as it was. Starting from this step no information about the delimiters is used in parsing. Interpolation The next step is interpolation in the text obtained, which is now delimiter-independent. There are four different cases. "<<'EOF'", "m''", "s'''", "tr///", "y///" No interpolation is performed. '', "q//" The only interpolation is removal of "\" from pairs "\\". "", ``, "qq//", "qx//", "" "\Q", "\U", "\u", "\L", "\l" (possibly paired with "\E") are converted to corresponding Perl constructs. Thus, "$foo\Qbaz$bar" is converted to "$foo . (quotemeta("baz" . $bar))" internally. The other combinations are replaced with appropriate expansions. Let it be stressed that whatever falls between "\Q" and "\E" is interpolated in the usual way. Something like "\Q\\E" has no "\E" inside. instead, it has "\Q", "\\", and "E", so the result is the same as for "\\\\E". As a general rule, back- slashes between "\Q" and "\E" may lead to counterintuitive results. So, "\Q\t\E" is converted to "quotemeta("\t")", which is the same as "\\\t" (since TAB is not alphanumeric). Note also that: $str = '\t'; return "\Q$str"; may be closer to the conjectural intention of the writer of "\Q\t\E". Interpolated scalars and arrays are converted internally to the "join" and "." catenation operations. Thus, "$foo XXX '@arr'" becomes: $foo . " XXX '" . (join $", @arr) . "'"; All operations above are performed simultaneously, left to right. Because the result of "\Q STRING \E" has all metacharacters quoted, there is no way to insert a literal "$" or "@" inside a "\Q\E" pair. If protected by "\", "$" will be quoted to became "\\\$"; if not, it is interpreted as the start of an interpo- lated scalar. Note also that the interpolation code needs to make a decision on where the interpolated scalar ends. For instance, whether "a $b -> {c}" really means: "a " . $b . " -> {c}"; or: "a " . $b -> {c}; Most of the time, the longest possible text that does not include spaces between components and which contains matching braces or brackets. because the outcome may be determined by voting based on heuristic estimators, the result is not strictly predictable. Fortunately, it's usually correct for ambiguous cases. "?RE?", "/RE/", "m/RE/", "s/RE/foo/", Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation happens (almost) as with "qq//" constructs, but the substitu- tion of "\" followed by RE-special chars (including "\") is not performed. Moreover, inside "(?{BLOCK})", "(?# comment )", and a "#"-comment in a "//x"-regular expression, no processing is performed whatsoever. This is the first step at which the presence of the "//x" modifier is relevant. Interpolation has several quirks: $|, $(, and $) are not inter- polated, and constructs $var[SOMETHING] are voted (by several different estimators) to be either an array element or $var followed by an RE alternative. This is where the notation "${arr[$bar]}" comes handy: "/${arr[0-9]}/" is interpreted as array element "-9", not as a regular expression from the vari- able $arr followed by a digit, which would be the interpreta- tion of "/$arr[0-9]/". Since voting among different estimators may occur, the result is not predictable. It is at this step that "\1" is begrudgingly converted to $1 in the replacement text of "s///" to correct the incorrigible sed hackers who haven't picked up the saner idiom yet. A warning is emitted if the "use warnings" pragma or the -w command-line flag (that is, the $^W variable) was set. The lack of processing of "\\" creates specific restrictions on the post-processed text. If the delimiter is "/", one cannot get the combination "\/" into the result of this step. "/" will finish the regular expression, "\/" will be stripped to "/" on the previous step, and "\\/" will be left as is. Because "/" is equivalent to "\/" inside a regular expression, this does not matter unless the delimiter happens to be charac- ter special to the RE engine, such as in "s*foo*bar*", "m[foo]", or "?foo?"; or an alphanumeric char, as in: m m ^ a \s* b mmx; In the RE above, which is intentionally obfuscated for illus- tration, the delimiter is "m", the modifier is "mx", and after backslash-removal the RE is the same as for "m/ ^ a \s* b /mx". There's more than one reason you're encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices. This step is the last one for all constructs except regular expres- sions, which are processed further. Interpolation of regular expressions Previous steps were performed during the compilation of Perl code, but this one happens at run time--although it may be optimized to be calculated at compile time if appropriate. After preprocessing described above, and possibly after evaluation if catenation, join- ing, casing translation, or metaquoting are involved, the resulting string is passed to the RE engine for compilation. Whatever happens in the RE engine might be better discussed in perlre, but for the sake of continuity, we shall do so here. This is another step where the presence of the "//x" modifier is relevant. The RE engine scans the string from left to right and converts it to a finite automaton. Backslashed characters are either replaced with corresponding lit- eral strings (as with "\{"), or else they generate special nodes in the finite automaton (as with "\b"). Characters special to the RE engine (such as "|") generate corresponding nodes or groups of nodes. "(?#...)" comments are ignored. All the rest is either converted to literal strings to match, or else is ignored (as is whitespace and "#"-style comments if "//x" is present). Parsing of the bracketed character class construct, "[...]", is rather different than the rule used for the rest of the pattern. The terminator of this construct is found using the same rules as for finding the terminator of a "{}"-delimited construct, the only exception being that "]" immediately following "[" is treated as though preceded by a backslash. Similarly, the terminator of "(?{...})" is found using the same rules as for finding the termi- nator of a "{}"-delimited construct. It is possible to inspect both the string given to RE engine and the resulting finite automaton. See the arguments "debug"/"debug- color" in the "use re" pragma, as well as Perl's -Dr command-line switch documented in "Command Switches" in perlrun. Optimization of regular expressions This step is listed for completeness only. Since it does not change semantics, details of this step are not documented and are subject to change without notice. This step is performed over the finite automaton that was generated during the previous pass. It is at this stage that "split()" silently optimizes "/^/" to mean "/^/m". I/O Operators There are several I/O operators you should know about. A string enclosed by backticks (grave accents) first undergoes double- quote interpolation. It is then interpreted as an external command, and the output of that command is the value of the backtick string, like in a shell. In scalar context, a single string consisting of all output is returned. In list context, a list of values is returned, one per line of output. (You can set $/ to use a different line termina- tor.) The command is executed each time the pseudo-literal is evalu- ated. The status value of the command is returned in $? (see perlvar for the interpretation of $?). Unlike in csh, no translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a backslash. The generalized form of backticks is "qx//". (Because backticks always undergo shell expansion as well, see perlsec for security concerns.) In scalar context, evaluating a filehandle in angle brackets yields the next line from that file (the newline, if any, included), or "undef" at end-of-file or on error. When $/ is set to "undef" (sometimes known as file-slurp mode) and the file is empty, it returns '' the first time, followed by "undef" subsequently. Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a "while" statement (even if disguised as a "for(;;)" loop), the value is auto- matically assigned to the global variable $_, destroying whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) The $_ vari- able is not implicitly localized. You'll have to put a "local $_;" before the loop if you want that to happen. The following lines are equivalent: while (defined($_ = )) { print; } while ($_ = ) { print; } while () { print; } for (;;) { print; } print while defined($_ = ); print while ($_ = ); print while ; This also behaves similarly, but avoids $_ : while (my $line = ) { print $line } In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly: while (($_ = ) ne '0') { ... } while () { last unless $_; ... } In other boolean contexts, "" without an explicit "defined" test or comparison elicit a warning if the "use warnings" pragma or the -w command-line switch (the $^W variable) is in effect. The filehandles STDIN, STDOUT, and STDERR are predefined. (The file- handles "stdin", "stdout", and "stderr" will also work except in pack- ages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() func- tion, amongst others. See perlopentut and "open" in perlfunc for details on this. If a is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. It's easy to grow to a rather large data space this way, so use with care. may also be spelled "readline(*FILEHANDLE)". See "read- line" in perlfunc. The null filehandle <> is special: it can be used to emulate the behav- ior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop while (<>) { ... # code for each line } is equivalent to the following Perl-like pseudo code: unshift(@ARGV, '-') unless @ARGV; while ($ARGV = shift) { open(ARGV, $ARGV); while () { ... # code for each line } } except that it isn't so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle ARGV internally--<> is just a synonym for , which is magical. (The pseudo code above doesn't work because it treats as non-magical.) You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) continue as though the input were one big happy file. See the example in "eof" in perlfunc for how to reset line numbers on each file. If you want to set @ARGV to your own list of files, go right ahead. This sets @ARGV to all plain text files if no @ARGV was given: @ARGV = grep { -f && -T } glob('*') unless @ARGV; You can even set them to pipe commands. For example, this automati- cally filters compressed arguments through gzip: @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this: while ($_ = $ARGV[0], /^-/) { shift; last if /^--$/; if (/^-D(.*)/) { $debug = $1 } if (/^-v/) { $verbose++ } # ... # other switches } while (<>) { # ... # code for each line } The <> symbol will return "undef" for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN. If what the angle brackets contain is a simple scalar variable (e.g., <$foo>), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example: $fh = \*STDIN; $line = <$fh>; If what's within the angle brackets is neither a filehandle nor a sim- ple scalar variable containing a filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and either a list of filenames or the next filename in the list is returned, depending on context. This distinction is determined on syn- tactic grounds alone. That means "<$x>" is always a readline() from an indirect handle, but "<$hash{key}>" is always a glob(). That's because $x is a simple scalar variable, but $hash{key} is not--it's a hash ele- ment. Even "<$x >" (note the extra space) is treated as "glob("$x ")", not "readline($x)". One level of double-quote interpretation is done first, but you can't say "<$foo>" because that's an indirect filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly brackets to force interpretation as a filename glob: "<${foo}>". These days, it's considered cleaner to call the internal function directly as "glob($foo)", which is probably the right way to have done it in the first place.) For example: while (<*.c>) { chmod 0644, $_; } is roughly equivalent to: open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); while () { chomp; chmod 0644, $_; } except that the globbing is actually done internally using the standard "File::Glob" extension. Of course, the shortest way to do the above is: chmod 0644, <*.c>; A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isn't important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time it's called, or "undef" when the list has run out. As with filehandle reads, an automatic "defined" is generated when the glob occurs in the test part of a "while", because legal glob returns (e.g. a file called 0) would otherwise terminate the loop. Again, "undef" is returned only once. So if you're expecting a single value from a glob, it is much better to say ($file) = ; than $file = ; because the latter will alternate between returning a filename and returning false. If you're trying to do variable interpolation, it's definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation. @files = glob("$dir/*.[ch]"); @files = glob($files[$i]); Constant Folding Like C, Perl does a certain amount of expression evaluation at compile time whenever it determines that all arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substi- tution. Backslash interpolation also happens at compile time. You can say 'Now is the time for all' . "\n" . 'good men to come to.' and this all reduces to one string internally. Likewise, if you say foreach $file (@filenames) { if (-s $file > 5 + 100 * 2**16) { } } the compiler will precompute the number which that expression repre- sents so that the interpreter won't have to. No-ops Perl doesn't officially have a no-op operator, but the bare constants 0 and 1 are special-cased to not produce a warning in a void context, so you can for example safely do 1 while foo(); Bitwise String Operators Bitstrings of any size may be manipulated by the bitwise operators ("~ | & ^"). If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated to the length of the shorter. The granularity for such extension or truncation is one or more bytes. # ASCII-based examples print "j p \n" ^ " a h"; # prints "JAPH\n" print "JA" | " ph\n"; # prints "japh\n" print "japh\nJunk" & '_____'; # prints "JAPH\n"; print 'p N$' ^ " Enumeric bitwise operation. You may explicitly show which type of oper- ation you intend by using "" or "0+", as in the examples below. $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) $foo = '150' | 105; # yields 255 $foo = 150 | '105'; # yields 255 $foo = '150' | '105'; # yields string '155' (under ASCII) $baz = 0+$foo & 0+$bar; # both ops explicitly numeric $biz = "$foo" ^ "$bar"; # both ops explicitly stringy See "vec" in perlfunc for information on how to manipulate individual bits in a bit vector. Integer Arithmetic By default, Perl assumes that it must do most of its arithmetic in floating point. But by saying use integer; you may tell the compiler that it's okay to use integer operations (if it feels like it) from here to the end of the enclosing BLOCK. An inner BLOCK may countermand this by saying no integer; which lasts until the end of that BLOCK. Note that this doesn't mean everything is only an integer, merely that Perl may use integer opera- tions if it is so inclined. For example, even under "use integer", if you take the sqrt(2), you'll still get 1.4142135623731 or so. Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", and ">>") always produce integral results. (But see also "Bitwise String Operators".) However, "use integer" still has meaning for them. By default, their results are interpreted as unsigned integers, but if "use integer" is in effect, their results are interpreted as signed integers. For example, "~0" usually evaluates to a large integral value. However, "use integer; ~0" is "-1" on twos-complement machines. Floating-point Arithmetic While "use integer" provides integer-only arithmetic, there is no anal- ogous mechanism to provide automatic rounding or truncation to a cer- tain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. See perl- faq4. Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. For example: printf "%.20g\n", 123456789123456789; # produces 123456789123456784 Testing for exact equality of floating-point equality or inequality is not a good idea. Here's a (relatively expensive) work-around to com- pare whether two floating-point numbers are equal to a particular num- ber of decimal places. See Knuth, volume II, for a more robust treat- ment of this topic. sub fp_equal { my ($X, $Y, $POINTS) = @_; my ($tX, $tY); $tX = sprintf("%.${POINTS}g", $X); $tY = sprintf("%.${POINTS}g", $Y); return $tX eq $tY; } The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and other mathematical and trigonometric functions. The Math::Complex module (part of the standard perl distribution) defines mathematical functions that work on both the reals and the imaginary numbers. Math::Complex not as efficient as POSIX, but POSIX can't work with complex numbers. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself. Bigger Numbers The standard Math::BigInt and Math::BigFloat modules provide variable- precision arithmetic and overloaded operators, although they're cur- rently pretty slow. At the cost of some space and considerable speed, they avoid the normal pitfalls associated with limited-precision repre- sentations. use Math::BigInt; $x = Math::BigInt->new('123456789123456789'); print $x * $x; # prints +15241578780673678515622620750190521 There are several modules that let you calculate with (bound only by memory and cpu-time) unlimited or fixed precision. There are also some non-standard modules that provide faster implementations via external C libraries. Here is a short, but incomplete summary: Math::Fraction big, unlimited fractions like 9973 / 12967 Math::String treat string sequences like numbers Math::FixedPrecision calculate with a fixed precision Math::Currency for currency calculations Bit::Vector manipulate bit vectors fast (uses C) Math::BigIntFast Bit::Vector wrapper for big numbers Math::Pari provides access to the Pari C library Math::BigInteger uses an external C library Math::Cephes uses external Cephes C library (no big numbers) Math::Cephes::Fraction fractions via the Cephes library Math::GMP another one using an external C library Choose wisely. perl v5.8.8 2006-01-07 PERLOP(1)
 
FreeBSD/Linux/UNIX General Commands Manual
Hypertext Man Pages
perlop
 
PERLOP(1)	       Perl Programmers Reference Guide 	     PERLOP(1)



NAME
       perlop - Perl operators and precedence

DESCRIPTION
       Operator Precedence and Associativity

       Operator precedence and associativity work in Perl more or less like
       they do in mathematics.

       Operator precedence means some operators are evaluated before others.
       For example, in "2 + 4 * 5", the multiplication has higher precedence
       so "4 * 5" is evaluated first yielding "2 + 20 == 22" and not "6 * 5 ==
       30".

       Operator associativity defines what happens if a sequence of the same
       operators is used one after another: whether the evaluator will evalu-
       ate the left operations first or the right.  For example, in "8 - 4 -
       2", subtraction is left associative so Perl evaluates the expression
       left to right.  "8 - 4" is evaluated first making the expression "4 - 2
       == 2" and not "8 - 2 == 6".

       Perl operators have the following associativity and precedence, listed
       from highest precedence to lowest.  Operators borrowed from C keep the
       same precedence relationship with each other, even where C's precedence
       is slightly screwy.  (This makes learning Perl easier for C folks.)
       With very few exceptions, these all operate on scalar values only, not
       array values.

	   left        terms and list operators (leftward)
	   left        ->
	   nonassoc    ++ --
	   right       **
	   right       ! ~ \ and unary + and -
	   left        =~ !~
	   left        * / % x
	   left        + - .
	   left        << >>
	   nonassoc    named unary operators
	   nonassoc    < > <= >= lt gt le ge
	   nonassoc    == != <=> eq ne cmp
	   left        &
	   left        | ^
	   left        &&
	   left        || //
	   nonassoc    ..  ...
	   right       ?:
	   right       = += -= *= etc.
	   left        , =>
	   nonassoc    list operators (rightward)
	   right       not
	   left        and
	   left        or xor err

       In the following sections, these operators are covered in precedence
       order.

       Many operators can be overloaded for objects.  See overload.

       Terms and List Operators (Leftward)

       A TERM has the highest precedence in Perl.  They include variables,
       quote and quote-like operators, any expression in parentheses, and any
       function whose arguments are parenthesized.  Actually, there aren't
       really functions in this sense, just list operators and unary operators
       behaving as functions because you put parentheses around the arguments.
       These are all documented in perlfunc.

       If any list operator (print(), etc.) or any unary operator (chdir(),
       etc.)  is followed by a left parenthesis as the next token, the opera-
       tor and arguments within parentheses are taken to be of highest prece-
       dence, just like a normal function call.

       In the absence of parentheses, the precedence of list operators such as
       "print", "sort", or "chmod" is either very high or very low depending
       on whether you are looking at the left side or the right side of the
       operator.  For example, in

	   @ary = (1, 3, sort 4, 2);
	   print @ary;	       # prints 1324

       the commas on the right of the sort are evaluated before the sort, but
       the commas on the left are evaluated after.  In other words, list oper-
       ators tend to gobble up all arguments that follow, and then act like a
       simple TERM with regard to the preceding expression.  Be careful with
       parentheses:

	   # These evaluate exit before doing the print:
	   print($foo, exit);  # Obviously not what you want.
	   print $foo, exit;   # Nor is this.

	   # These do the print before evaluating exit:
	   (print $foo), exit; # This is what you want.
	   print($foo), exit;  # Or this.
	   print ($foo), exit; # Or even this.

       Also note that

	   print ($foo & 255) + 1, "\n";

       probably doesn't do what you expect at first glance.  The parentheses
       enclose the argument list for "print" which is evaluated (printing the
       result of "$foo & 255").  Then one is added to the return value of
       "print" (usually 1).  The result is something like this:

	   1 + 1, "\n";    # Obviously not what you meant.

       To do what you meant properly, you must write:

	   print(($foo & 255) + 1, "\n");

       See "Named Unary Operators" for more discussion of this.

       Also parsed as terms are the "do {}" and "eval {}" constructs, as well
       as subroutine and method calls, and the anonymous constructors "[]" and
       "{}".

       See also "Quote and Quote-like Operators" toward the end of this sec-
       tion, as well as "I/O Operators".

       The Arrow Operator

       ""->"" is an infix dereference operator, just as it is in C and C++.
       If the right side is either a "[...]", "{...}", or a "(...)" subscript,
       then the left side must be either a hard or symbolic reference to an
       array, a hash, or a subroutine respectively.  (Or technically speaking,
       a location capable of holding a hard reference, if it's an array or
       hash reference being used for assignment.)  See perlreftut and perlref.

       Otherwise, the right side is a method name or a simple scalar variable
       containing either the method name or a subroutine reference, and the
       left side must be either an object (a blessed reference) or a class
       name (that is, a package name).	See perlobj.

       Auto-increment and Auto-decrement

       "++" and "--" work as in C.  That is, if placed before a variable, they
       increment or decrement the variable by one before returning the value,
       and if placed after, increment or decrement after returning the value.

	   $i = 0;  $j = 0;
	   print $i++;	# prints 0
	   print ++$j;	# prints 1

       Note that just as in C, Perl doesn't define when the variable is incre-
       mented or decremented. You just know it will be done sometime before or
       after the value is returned. This also means that modifying a variable
       twice in the same statement will lead to undefined behaviour.  Avoid
       statements like:

	   $i = $i ++;
	   print ++ $i + $i ++;

       Perl will not guarantee what the result of the above statements is.

       The auto-increment operator has a little extra builtin magic to it.  If
       you increment a variable that is numeric, or that has ever been used in
       a numeric context, you get a normal increment.  If, however, the vari-
       able has been used in only string contexts since it was set, and has a
       value that is not the empty string and matches the pattern
       "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
       each character within its range, with carry:

	   print ++($foo = '99');      # prints '100'
	   print ++($foo = 'a0');      # prints 'a1'
	   print ++($foo = 'Az');      # prints 'Ba'
	   print ++($foo = 'zz');      # prints 'aaa'

       "undef" is always treated as numeric, and in particular is changed to 0
       before incrementing (so that a post-increment of an undef value will
       return 0 rather than "undef").

       The auto-decrement operator is not magical.

       Exponentiation

       Binary "**" is the exponentiation operator.  It binds even more tightly
       than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is imple-
       mented using C's pow(3) function, which actually works on doubles
       internally.)

       Symbolic Unary Operators

       Unary "!" performs logical negation, i.e., "not".  See also "not" for a
       lower precedence version of this.

       Unary "-" performs arithmetic negation if the operand is numeric.  If
       the operand is an identifier, a string consisting of a minus sign con-
       catenated with the identifier is returned.  Otherwise, if the string
       starts with a plus or minus, a string starting with the opposite sign
       is returned.  One effect of these rules is that -bareword is equivalent
       to the string "-bareword".  If, however, the string begins with a non-
       alphabetic character (exluding "+" or "-"), Perl will attempt to con-
       vert the string to a numeric and the arithmetic negation is performed.
       If the string cannot be cleanly converted to a numeric, Perl will give
       the warning Argument "the string" isn't numeric in negation (-) at ....

       Unary "~" performs bitwise negation, i.e., 1's complement.  For exam-
       ple, "0666 & ~027" is 0640.  (See also "Integer Arithmetic" and "Bit-
       wise String Operators".)  Note that the width of the result is plat-
       form-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits
       wide on a 64-bit platform, so if you are expecting a certain bit width,
       remember to use the & operator to mask off the excess bits.

       Unary "+" has no effect whatsoever, even on strings.  It is useful syn-
       tactically for separating a function name from a parenthesized expres-
       sion that would otherwise be interpreted as the complete list of func-
       tion arguments.	(See examples above under "Terms and List Operators
       (Leftward)".)

       Unary "\" creates a reference to whatever follows it.  See perlreftut
       and perlref.  Do not confuse this behavior with the behavior of back-
       slash within a string, although both forms do convey the notion of pro-
       tecting the next thing from interpolation.

       Binding Operators

       Binary "=~" binds a scalar expression to a pattern match.  Certain
       operations search or modify the string $_ by default.  This operator
       makes that kind of operation work on some other string.	The right
       argument is a search pattern, substitution, or transliteration.	The
       left argument is what is supposed to be searched, substituted, or
       transliterated instead of the default $_.  When used in scalar context,
       the return value generally indicates the success of the operation.
       Behavior in list context depends on the particular operator.  See "Reg-
       exp Quote-Like Operators" for details and perlretut for examples using
       these operators.

       If the right argument is an expression rather than a search pattern,
       substitution, or transliteration, it is interpreted as a search pattern
       at run time.

       Binary "!~" is just like "=~" except the return value is negated in the
       logical sense.

       Multiplicative Operators

       Binary "*" multiplies two numbers.

       Binary "/" divides two numbers.

       Binary "%" computes the modulus of two numbers.	Given integer operands
       $a and $b: If $b is positive, then "$a % $b" is $a minus the largest
       multiple of $b that is not greater than $a.  If $b is negative, then
       "$a % $b" is $a minus the smallest multiple of $b that is not less than
       $a (i.e. the result will be less than or equal to zero).  Note that
       when "use integer" is in scope, "%" gives you direct access to the mod-
       ulus operator as implemented by your C compiler.  This operator is not
       as well defined for negative operands, but it will execute faster.

       Binary "x" is the repetition operator.  In scalar context or if the
       left operand is not enclosed in parentheses, it returns a string con-
       sisting of the left operand repeated the number of times specified by
       the right operand.  In list context, if the left operand is enclosed in
       parentheses or is a list formed by "qw/STRING/", it repeats the list.
       If the right operand is zero or negative, it returns an empty string or
       an empty list, depending on the context.

	   print '-' x 80;	       # print row of dashes

	   print "\t" x ($tab/8), ' ' x ($tab%8);      # tab over

	   @ones = (1) x 80;	       # a list of 80 1's
	   @ones = (5) x @ones;        # set all elements to 5

       Additive Operators

       Binary "+" returns the sum of two numbers.

       Binary "-" returns the difference of two numbers.

       Binary "." concatenates two strings.

       Shift Operators

       Binary "<<" returns the value of its left argument shifted left by the
       number of bits specified by the right argument.	Arguments should be
       integers.  (See also "Integer Arithmetic".)

       Binary ">>" returns the value of its left argument shifted right by the
       number of bits specified by the right argument.	Arguments should be
       integers.  (See also "Integer Arithmetic".)

       Note that both "<<" and ">>" in Perl are implemented directly using
       "<<" and ">>" in C.  If "use integer" (see "Integer Arithmetic") is in
       force then signed C integers are used, else unsigned C integers are
       used.  Either way, the implementation isn't going to generate results
       larger than the size of the integer type Perl was built with (32 bits
       or 64 bits).

       The result of overflowing the range of the integers is undefined
       because it is undefined also in C.  In other words, using 32-bit inte-
       gers, "1 << 32" is undefined.  Shifting by a negative number of bits is
       also undefined.

       Named Unary Operators

       The various named unary operators are treated as functions with one
       argument, with optional parentheses.

       If any list operator (print(), etc.) or any unary operator (chdir(),
       etc.)  is followed by a left parenthesis as the next token, the opera-
       tor and arguments within parentheses are taken to be of highest prece-
       dence, just like a normal function call.  For example, because named
       unary operators are higher precedence than ||:

	   chdir $foo	 || die;       # (chdir $foo) || die
	   chdir($foo)	 || die;       # (chdir $foo) || die
	   chdir ($foo)  || die;       # (chdir $foo) || die
	   chdir +($foo) || die;       # (chdir $foo) || die

       but, because * is higher precedence than named operators:

	   chdir $foo * 20;    # chdir ($foo * 20)
	   chdir($foo) * 20;   # (chdir $foo) * 20
	   chdir ($foo) * 20;  # (chdir $foo) * 20
	   chdir +($foo) * 20; # chdir ($foo * 20)

	   rand 10 * 20;       # rand (10 * 20)
	   rand(10) * 20;      # (rand 10) * 20
	   rand (10) * 20;     # (rand 10) * 20
	   rand +(10) * 20;    # rand (10 * 20)

       Regarding precedence, the filetest operators, like "-f", "-M", etc. are
       treated like named unary operators, but they don't follow this func-
       tional parenthesis rule.  That means, for example, that
       "-f($file).".bak"" is equivalent to "-f "$file.bak"".

       See also "Terms and List Operators (Leftward)".

       Relational Operators

       Binary "<" returns true if the left argument is numerically less than
       the right argument.

       Binary ">" returns true if the left argument is numerically greater
       than the right argument.

       Binary "<=" returns true if the left argument is numerically less than
       or equal to the right argument.

       Binary ">=" returns true if the left argument is numerically greater
       than or equal to the right argument.

       Binary "lt" returns true if the left argument is stringwise less than
       the right argument.

       Binary "gt" returns true if the left argument is stringwise greater
       than the right argument.

       Binary "le" returns true if the left argument is stringwise less than
       or equal to the right argument.

       Binary "ge" returns true if the left argument is stringwise greater
       than or equal to the right argument.

       Equality Operators

       Binary "==" returns true if the left argument is numerically equal to
       the right argument.

       Binary "!=" returns true if the left argument is numerically not equal
       to the right argument.

       Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
       is numerically less than, equal to, or greater than the right argument.
       If your platform supports NaNs (not-a-numbers) as numeric values, using
       them with "<=>" returns undef.  NaN is not "<", "==", ">", "<=" or ">="
       anything (even NaN), so those 5 return false. NaN != NaN returns true,
       as does NaN != anything else. If your platform doesn't support NaNs
       then NaN is just a string with numeric value 0.

	   perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
	   perl -le '$a = "NaN"; print "NaN support here" if $a != $a'

       Binary "eq" returns true if the left argument is stringwise equal to
       the right argument.

       Binary "ne" returns true if the left argument is stringwise not equal
       to the right argument.

       Binary "cmp" returns -1, 0, or 1 depending on whether the left argument
       is stringwise less than, equal to, or greater than the right argument.

       "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order speci-
       fied by the current locale if "use locale" is in effect.  See perllo-
       cale.

       Bitwise And

       Binary "&" returns its operands ANDed together bit by bit.  (See also
       "Integer Arithmetic" and "Bitwise String Operators".)

       Note that "&" has lower priority than relational operators, so for
       example the brackets are essential in a test like

	       print "Even\n" if ($x & 1) == 0;

       Bitwise Or and Exclusive Or

       Binary "|" returns its operands ORed together bit by bit.  (See also
       "Integer Arithmetic" and "Bitwise String Operators".)

       Binary "^" returns its operands XORed together bit by bit.  (See also
       "Integer Arithmetic" and "Bitwise String Operators".)

       Note that "|" and "^" have lower priority than relational operators, so
       for example the brackets are essential in a test like

	       print "false\n" if (8 | 2) != 10;

       C-style Logical And

       Binary "&&" performs a short-circuit logical AND operation.  That is,
       if the left operand is false, the right operand is not even evaluated.
       Scalar or list context propagates down to the right operand if it is
       evaluated.

       C-style Logical Defined-Or

       Although it has no direct equivalent in C, Perl's "//" operator is
       related to its C-style or.  In fact, it's exactly the same as "||",
       except that it tests the left hand side's definedness instead of its
       truth.  Thus, "$a // $b" is similar to "defined($a) || $b" (except that
       it returns the value of $a rather than the value of "defined($a)") and
       is exactly equivalent to "defined($a) ? $a : $b".  This is very useful
       for providing default values for variables.  If you actually want to
       test if at least one of $a and $b is defined, use "defined($a // $b)".

       C-style Logical Or

       Binary "||" performs a short-circuit logical OR operation.  That is, if
       the left operand is true, the right operand is not even evaluated.
       Scalar or list context propagates down to the right operand if it is
       evaluated.

       The "||", "//" and "&&" operators return the last value evaluated
       (unlike C's "||" and "&&", which return 0 or 1). Thus, a reasonably
       portable way to find out the home directory might be:

	   $home = $ENV{HOME} // $ENV{LOGDIR} //
	       (getpwuid ($<))[7] // die "You're homeless!\n";

       In particular, this means that you shouldn't use this for selecting
       between two aggregates for assignment:

	   @a = @b || @c;	       # this is wrong
	   @a = scalar(@b) || @c;      # really meant this
	   @a = @b ? @b : @c;	       # this works fine, though

       As more readable alternatives to "&&", "//" and "||" when used for con-
       trol flow, Perl provides "and", "err" and "or" operators (see below).
       The short-circuit behavior is identical.  The precedence of "and",
       "err" and "or" is much lower, however, so that you can safely use them
       after a list operator without the need for parentheses:

	   unlink "alpha", "beta", "gamma"
		   or gripe(), next LINE;

       With the C-style operators that would have been written like this:

	   unlink("alpha", "beta", "gamma")
		   || (gripe(), next LINE);

       Using "or" for assignment is unlikely to do what you want; see below.

       Range Operators

       Binary ".." is the range operator, which is really two different opera-
       tors depending on the context.  In list context, it returns a list of
       values counting (up by ones) from the left value to the right value.
       If the left value is greater than the right value then it returns the
       empty list.  The range operator is useful for writing "foreach (1..10)"
       loops and for doing slice operations on arrays. In the current imple-
       mentation, no temporary array is created when the range operator is
       used as the expression in "foreach" loops, but older versions of Perl
       might burn a lot of memory when you write something like this:

	   for (1 .. 1_000_000) {
	       # code
	   }

       The range operator also works on strings, using the magical auto-incre-
       ment, see below.

       In scalar context, ".." returns a boolean value.  The operator is
       bistable, like a flip-flop, and emulates the line-range (comma) opera-
       tor of sed, awk, and various editors.  Each ".." operator maintains its
       own boolean state.  It is false as long as its left operand is false.
       Once the left operand is true, the range operator stays true until the
       right operand is true, AFTER which the range operator becomes false
       again.  It doesn't become false till the next time the range operator
       is evaluated.  It can test the right operand and become false on the
       same evaluation it became true (as in awk), but it still returns true
       once.  If you don't want it to test the right operand till the next
       evaluation, as in sed, just use three dots ("...") instead of two.  In
       all other regards, "..." behaves just like ".." does.

       The right operand is not evaluated while the operator is in the "false"
       state, and the left operand is not evaluated while the operator is in
       the "true" state.  The precedence is a little lower than || and &&.
       The value returned is either the empty string for false, or a sequence
       number (beginning with 1) for true.  The sequence number is reset for
       each range encountered.	The final sequence number in a range has the
       string "E0" appended to it, which doesn't affect its numeric value, but
       gives you something to search for if you want to exclude the endpoint.
       You can exclude the beginning point by waiting for the sequence number
       to be greater than 1.

       If either operand of scalar ".." is a constant expression, that operand
       is considered true if it is equal ("==") to the current input line num-
       ber (the $. variable).

       To be pedantic, the comparison is actually "int(EXPR) == int(EXPR)",
       but that is only an issue if you use a floating point expression; when
       implicitly using $. as described in the previous paragraph, the compar-
       ison is "int(EXPR) == int($.)" which is only an issue when $.  is set
       to a floating point value and you are not reading from a file.  Fur-
       thermore, "span" .. "spat" or "2.18 .. 3.14" will not do what you want
       in scalar context because each of the operands are evaluated using
       their integer representation.

       Examples:

       As a scalar operator:

	   if (101 .. 200) { print; } # print 2nd hundred lines, short for
				      #   if ($. == 101 .. $. == 200) ...

	   next LINE if (1 .. /^$/);  # skip header lines, short for
				      #   ... if ($. == 1 .. /^$/);
				      # (typically in a loop labeled LINE)

	   s/^/> / if (/^$/ .. eof());	# quote body

	   # parse mail messages
	   while (<>) {
	       $in_header =   1  .. /^$/;
	       $in_body   = /^$/ .. eof;
	       if ($in_header) {
		   # ...
	       } else { # in body
		   # ...
	       }
	   } continue {
	       close ARGV if eof;	      # reset $. each file
	   }

       Here's a simple example to illustrate the difference between the two
       range operators:

	   @lines = ("	 - Foo",
		     "01 - Bar",
		     "1  - Baz",
		     "	 - Quux");

	   foreach (@lines) {
	       if (/0/ .. /1/) {
		   print "$_\n";
	       }
	   }

       This program will print only the line containing "Bar". If the range
       operator is changed to "...", it will also print the "Baz" line.

       And now some examples as a list operator:

	   for (101 .. 200) { print; } # print $_ 100 times
	   @foo = @foo[0 .. $#foo];    # an expensive no-op
	   @foo = @foo[$#foo-4 .. $#foo];      # slice last 5 items

       The range operator (in list context) makes use of the magical auto-
       increment algorithm if the operands are strings.  You can say

	   @alphabet = ('A' .. 'Z');

       to get all normal letters of the English alphabet, or

	   $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

       to get a hexadecimal digit, or

	   @z2 = ('01' .. '31');  print $z2[$mday];

       to get dates with leading zeros.  If the final value specified is not
       in the sequence that the magical increment would produce, the sequence
       goes until the next value would be longer than the final value speci-
       fied.

       Because each operand is evaluated in integer form, "2.18 .. 3.14" will
       return two elements in list context.

	   @list = (2.18 .. 3.14); # same as @list = (2 .. 3);

       Conditional Operator

       Ternary "?:" is the conditional operator, just as in C.	It works much
       like an if-then-else.  If the argument before the ? is true, the argu-
       ment before the : is returned, otherwise the argument after the : is
       returned.  For example:

	   printf "I have %d dog%s.\n", $n,
		   ($n == 1) ? '' : "s";

       Scalar or list context propagates downward into the 2nd or 3rd argu-
       ment, whichever is selected.

	   $a = $ok ? $b : $c;	# get a scalar
	   @a = $ok ? @b : @c;	# get an array
	   $a = $ok ? @b : @c;	# oops, that's just a count!

       The operator may be assigned to if both the 2nd and 3rd arguments are
       legal lvalues (meaning that you can assign to them):

	   ($a_or_b ? $a : $b) = $c;

       Because this operator produces an assignable result, using assignments
       without parentheses will get you in trouble.  For example, this:

	   $a % 2 ? $a += 10 : $a += 2

       Really means this:

	   (($a % 2) ? ($a += 10) : $a) += 2

       Rather than this:

	   ($a % 2) ? ($a += 10) : ($a += 2)

       That should probably be written more simply as:

	   $a += ($a % 2) ? 10 : 2;

       Assignment Operators

       "=" is the ordinary assignment operator.

       Assignment operators work as in C.  That is,

	   $a += 2;

       is equivalent to

	   $a = $a + 2;

       although without duplicating any side effects that dereferencing the
       lvalue might trigger, such as from tie().  Other assignment operators
       work similarly.	The following are recognized:

	   **=	  +=	*=    &=    <<=    &&=
		  -=	/=    |=    >>=    ||=
		  .=	%=    ^=	   //=
			x=

       Although these are grouped by family, they all have the precedence of
       assignment.

       Unlike in C, the scalar assignment operator produces a valid lvalue.
       Modifying an assignment is equivalent to doing the assignment and then
       modifying the variable that was assigned to.  This is useful for modi-
       fying a copy of something, like this:

	   ($tmp = $global) =~ tr [A-Z] [a-z];

       Likewise,

	   ($a += 2) *= 3;

       is equivalent to

	   $a += 2;
	   $a *= 3;

       Similarly, a list assignment in list context produces the list of lval-
       ues assigned to, and a list assignment in scalar context returns the
       number of elements produced by the expression on the right hand side of
       the assignment.

       Comma Operator

       Binary "," is the comma operator.  In scalar context it evaluates its
       left argument, throws that value away, then evaluates its right argu-
       ment and returns that value.  This is just like C's comma operator.

       In list context, it's just the list argument separator, and inserts
       both its arguments into the list.

       The "=>" operator is a synonym for the comma, but forces any word (con-
       sisting entirely of word characters) to its left to be interpreted as a
       string (as of 5.001).  This includes words that might otherwise be con-
       sidered a constant or function call.

	   use constant FOO => "something";

	   my %h = ( FOO => 23 );

       is equivalent to:

	   my %h = ("FOO", 23);

       It is NOT:

	   my %h = ("something", 23);

       If the argument on the left is not a word, it is first interpreted as
       an expression, and then the string value of that is used.

       The "=>" operator is helpful in documenting the correspondence between
       keys and values in hashes, and other paired elements in lists.

	       %hash = ( $key => $value );
	       login( $username => $password );

       List Operators (Rightward)

       On the right side of a list operator, it has very low precedence, such
       that it controls all comma-separated expressions found there.  The only
       operators with lower precedence are the logical operators "and", "or",
       and "not", which may be used to evaluate calls to list operators with-
       out the need for extra parentheses:

	   open HANDLE, "filename"
	       or die "Can't open: $!\n";

       See also discussion of list operators in "Terms and List Operators
       (Leftward)".

       Logical Not

       Unary "not" returns the logical negation of the expression to its
       right.  It's the equivalent of "!" except for the very low precedence.

       Logical And

       Binary "and" returns the logical conjunction of the two surrounding
       expressions.  It's equivalent to && except for the very low precedence.
       This means that it short-circuits: i.e., the right expression is evalu-
       ated only if the left expression is true.

       Logical or, Defined or, and Exclusive Or

       Binary "or" returns the logical disjunction of the two surrounding
       expressions.  It's equivalent to || except for the very low precedence.
       This makes it useful for control flow

	   print FH $data	       or die "Can't write to FH: $!";

       This means that it short-circuits: i.e., the right expression is evalu-
       ated only if the left expression is false.  Due to its precedence, you
       should probably avoid using this for assignment, only for control flow.

	   $a = $b or $c;	       # bug: this is wrong
	   ($a = $b) or $c;	       # really means this
	   $a = $b || $c;	       # better written this way

       However, when it's a list-context assignment and you're trying to use
       "||" for control flow, you probably need "or" so that the assignment
       takes higher precedence.

	   @info = stat($file) || die;	   # oops, scalar sense of stat!
	   @info = stat($file) or die;	   # better, now @info gets its due

       Then again, you could always use parentheses.

       Binary "err" is equivalent to "//"--it's just like binary "or", except
       it tests its left argument's definedness instead of its truth.  There
       are two ways to remember "err":	either because many functions return
       "undef" on an error, or as a sort of correction:  "$a=($b err
       'default')"

       Binary "xor" returns the exclusive-OR of the two surrounding expres-
       sions.  It cannot short circuit, of course.

       C Operators Missing From Perl

       Here is what C has that Perl doesn't:

       unary & Address-of operator.  (But see the "\" operator for taking a
	       reference.)

       unary * Dereference-address operator. (Perl's prefix dereferencing
	       operators are typed: $, @, %, and &.)

       (TYPE)  Type-casting operator.

       Quote and Quote-like Operators

       While we usually think of quotes as literal values, in Perl they func-
       tion as operators, providing various kinds of interpolating and pattern
       matching capabilities.  Perl provides customary quote characters for
       these behaviors, but also provides a way for you to choose your quote
       character for any of them.  In the following table, a "{}" represents
       any pair of delimiters you choose.

	   Customary  Generic	     Meaning	    Interpolates
	       ''	q{}	     Literal		 no
	       ""      qq{}	     Literal		 yes
	       ``      qx{}	     Command		 yes*
		       qw{}	    Word list		 no
	       //	m{}	  Pattern match 	 yes*
		       qr{}	     Pattern		 yes*
			s{}{}	   Substitution 	 yes*
		       tr{}{}	 Transliteration	 no (but see below)
	       <NOTE: Unlike C and other languages, Perl has no \v escape sequence for
       the vertical tab (VT - ASCII 11).

       The following escape sequences are available in constructs that inter-
       polate but not in transliterations.

	   \l	       lowercase next char
	   \u	       uppercase next char
	   \L	       lowercase till \E
	   \U	       uppercase till \E
	   \E	       end case modification
	   \Q	       quote non-word characters till \E

       If "use locale" is in effect, the case map used by "\l", "\L", "\u" and
       "\U" is taken from the current locale.  See perllocale.	If Unicode
       (for example, "\N{}" or wide hex characters of 0x100 or beyond) is
       being used, the case map used by "\l", "\L", "\u" and "\U" is as
       defined by Unicode.  For documentation of "\N{name}", see charnames.

       All systems use the virtual "\n" to represent a line terminator, called
       a "newline".  There is no such thing as an unvarying, physical newline
       character.  It is only an illusion that the operating system, device
       drivers, C libraries, and Perl all conspire to preserve.  Not all sys-
       tems read "\r" as ASCII CR and "\n" as ASCII LF.  For example, on a
       Mac, these are reversed, and on systems without line terminator, print-
       ing "\n" may emit no actual data.  In general, use "\n" when you mean a
       "newline" for your system, but use the literal ASCII when you need an
       exact character.  For example, most networking protocols expect and
       prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators, and
       although they often accept just "\012", they seldom tolerate just
       "\015".	If you get in the habit of using "\n" for networking, you may
       be burned some day.

       For constructs that do interpolate, variables beginning with ""$"" or
       ""@"" are interpolated.	Subscripted variables such as $a[3] or
       "$href->{key}[0]" are also interpolated, as are array and hash slices.
       But method calls such as "$obj->meth" are not.

       Interpolating an array or slice interpolates the elements in order,
       separated by the value of $", so is equivalent to interpolating "join
       $", @array".    "Punctuation" arrays such as "@+" are only interpolated
       if the name is enclosed in braces "@{+}".

       You cannot include a literal "$" or "@" within a "\Q" sequence.	An
       unescaped "$" or "@" interpolates the corresponding variable, while
       escaping will cause the literal string "\$" to be inserted.  You'll
       need to write something like "m/\Quser\E\@\Qhost/".

       Patterns are subject to an additional level of interpretation as a reg-
       ular expression.  This is done as a second pass, after variables are
       interpolated, so that regular expressions may be incorporated into the
       pattern from the variables.  If this is not what you want, use "\Q" to
       interpolate a variable literally.

       Apart from the behavior described above, Perl does not expand multiple
       levels of interpolation.  In particular, contrary to the expectations
       of shell programmers, back-quotes do NOT interpolate within double
       quotes, nor do single quotes impede evaluation of variables when used
       within double quotes.

       Regexp Quote-Like Operators

       Here are the quote-like operators that apply to pattern matching and
       related activities.

       ?PATTERN?
	       This is just like the "/pattern/" search, except that it
	       matches only once between calls to the reset() operator.  This
	       is a useful optimization when you want to see only the first
	       occurrence of something in each file of a set of files, for
	       instance.  Only "??"  patterns local to the current package are
	       reset.

		   while (<>) {
		       if (?^$?) {
					   # blank line between header and body
		       }
		   } continue {
		       reset if eof;	   # clear ?? status for next file
		   }

	       This usage is vaguely deprecated, which means it just might
	       possibly be removed in some distant future version of Perl,
	       perhaps somewhere around the year 2168.

       m/PATTERN/cgimosx
       /PATTERN/cgimosx
	       Searches a string for a pattern match, and in scalar context
	       returns true if it succeeds, false if it fails.	If no string
	       is specified via the "=~" or "!~" operator, the $_ string is
	       searched.  (The string specified with "=~" need not be an
	       lvalue--it may be the result of an expression evaluation, but
	       remember the "=~" binds rather tightly.)  See also perlre.  See
	       perllocale for discussion of additional considerations that
	       apply when "use locale" is in effect.

	       Options are:

		   c   Do not reset search position on a failed match when /g is in effect.
		   g   Match globally, i.e., find all occurrences.
		   i   Do case-insensitive pattern matching.
		   m   Treat string as multiple lines.
		   o   Compile pattern only once.
		   s   Treat string as single line.
		   x   Use extended regular expressions.

	       If "/" is the delimiter then the initial "m" is optional.  With
	       the "m" you can use any pair of non-alphanumeric, non-white-
	       space characters as delimiters.	This is particularly useful
	       for matching path names that contain "/", to avoid LTS (leaning
	       toothpick syndrome).  If "?" is the delimiter, then the match-
	       only-once rule of "?PATTERN?" applies.  If "'" is the delim-
	       iter, no interpolation is performed on the PATTERN.

	       PATTERN may contain variables, which will be interpolated (and
	       the pattern recompiled) every time the pattern search is evalu-
	       ated, except for when the delimiter is a single quote.  (Note
	       that $(, $), and $| are not interpolated because they look like
	       end-of-string tests.)  If you want such a pattern to be com-
	       piled only once, add a "/o" after the trailing delimiter.  This
	       avoids expensive run-time recompilations, and is useful when
	       the value you are interpolating won't change over the life of
	       the script.  However, mentioning "/o" constitutes a promise
	       that you won't change the variables in the pattern.  If you
	       change them, Perl won't even notice.  See also
	       "qr/STRING/imosx".

	       If the PATTERN evaluates to the empty string, the last success-
	       fully matched regular expression is used instead. In this case,
	       only the "g" and "c" flags on the empty pattern is honoured -
	       the other flags are taken from the original pattern. If no
	       match has previously succeeded, this will (silently) act
	       instead as a genuine empty pattern (which will always match).

	       Note that it's possible to confuse Perl into thinking "//" (the
	       empty regex) is really "//" (the defined-or operator).  Perl is
	       usually pretty good about this, but some pathological cases
	       might trigger this, such as "$a///" (is that "($a) / (//)" or
	       "$a // /"?) and "print $fh //" ("print $fh(//" or "print($fh
	       //"?).  In all of these examples, Perl will assume you meant
	       defined-or.  If you meant the empty regex, just use parentheses
	       or spaces to disambiguate, or even prefix the empty regex with
	       an "m" (so "//" becomes "m//").

	       If the "/g" option is not used, "m//" in list context returns a
	       list consisting of the subexpressions matched by the parenthe-
	       ses in the pattern, i.e., ($1, $2, $3...).  (Note that here $1
	       etc. are also set, and that this differs from Perl 4's behav-
	       ior.)  When there are no parentheses in the pattern, the return
	       value is the list "(1)" for success.  With or without parenthe-
	       ses, an empty list is returned upon failure.

	       Examples:

		   open(TTY, '/dev/tty');
		    =~ /^y/i && foo();    # do foo if desired

		   if (/Version: *([0-9.]*)/) { $version = $1; }

		   next if m#^/usr/spool/uucp#;

		   # poor man's grep
		   $arg = shift;
		   while (<>) {
		       print if /$arg/o;       # compile only once
		   }

		   if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

	       This last example splits $foo into the first two words and the
	       remainder of the line, and assigns those three fields to $F1,
	       $F2, and $Etc.  The conditional is true if any variables were
	       assigned, i.e., if the pattern matched.

	       The "/g" modifier specifies global pattern matching--that is,
	       matching as many times as possible within the string.  How it
	       behaves depends on the context.	In list context, it returns a
	       list of the substrings matched by any capturing parentheses in
	       the regular expression.	If there are no parentheses, it
	       returns a list of all the matched strings, as if there were
	       parentheses around the whole pattern.

	       In scalar context, each execution of "m//g" finds the next
	       match, returning true if it matches, and false if there is no
	       further match.  The position after the last match can be read
	       or set using the pos() function; see "pos" in perlfunc.	 A
	       failed match normally resets the search position to the begin-
	       ning of the string, but you can avoid that by adding the "/c"
	       modifier (e.g. "m//gc").  Modifying the target string also
	       resets the search position.

	       You can intermix "m//g" matches with "m/\G.../g", where "\G" is
	       a zero-width assertion that matches the exact position where
	       the previous "m//g", if any, left off.  Without the "/g" modi-
	       fier, the "\G" assertion still anchors at pos(), but the match
	       is of course only attempted once.  Using "\G" without "/g" on a
	       target string that has not previously had a "/g" match applied
	       to it is the same as using the "\A" assertion to match the
	       beginning of the string.  Note also that, currently, "\G" is
	       only properly supported when anchored at the very beginning of
	       the pattern.

	       Examples:

		   # list context
		   ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

		   # scalar context
		   $/ = "";
		   while (defined($paragraph = <>)) {
		       while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
			   $sentences++;
		       }
		   }
		   print "$sentences\n";

		   # using m//gc with \G
		   $_ = "ppooqppqq";
		   while ($i++ < 2) {
		       print "1: '";
		       print $1 while /(o)/gc; print "', pos=", pos, "\n";
		       print "2: '";
		       print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
		       print "3: '";
		       print $1 while /(p)/gc; print "', pos=", pos, "\n";
		   }
		   print "Final: '$1', pos=",pos,"\n" if /\G(.)/;

	       The last example should print:

		   1: 'oo', pos=4
		   2: 'q', pos=5
		   3: 'pp', pos=7
		   1: '', pos=7
		   2: 'q', pos=8
		   3: '', pos=8
		   Final: 'q', pos=8

	       Notice that the final match matched "q" instead of "p", which a
	       match without the "\G" anchor would have done. Also note that
	       the final match did not update "pos" -- "pos" is only updated
	       on a "/g" match. If the final match did indeed match "p", it's
	       a good bet that you're running an older (pre-5.6.0) Perl.

	       A useful idiom for "lex"-like scanners is "/\G.../gc".  You can
	       combine several regexps like this to process a string
	       part-by-part, doing different actions depending on which regexp
	       matched.  Each regexp tries to match where the previous one
	       leaves off.

		$_ = <<'EOL';
		     $url = new URI::URL "http://www/";   die if $url eq "xXx";
		EOL
		LOOP:
		   {
		     print(" digits"),	       redo LOOP if /\G\d+\b[,.;]?\s*/gc;
		     print(" lowercase"),      redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
		     print(" UPPERCASE"),      redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
		     print(" Capitalized"),    redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
		     print(" MiXeD"),	       redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
		     print(" alphanumeric"),   redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
		     print(" line-noise"),     redo LOOP if /\G[^A-Za-z0-9]+/gc;
		     print ". That's all!\n";
		   }

	       Here is the output (split into several lines):

		line-noise lowercase line-noise lowercase UPPERCASE line-noise
		UPPERCASE line-noise lowercase line-noise lowercase line-noise
		lowercase lowercase line-noise lowercase lowercase line-noise
		MiXeD line-noise. That's all!

       q/STRING/
       'STRING'
	       A single-quoted, literal string.  A backslash represents a
	       backslash unless followed by the delimiter or another back-
	       slash, in which case the delimiter or backslash is interpo-
	       lated.

		   $foo = q!I said, "You said, 'She said it.'"!;
		   $bar = q('This is it.');
		   $baz = '\n'; 	       # a two-character string

       qq/STRING/
       "STRING"
	       A double-quoted, interpolated string.

		   $_ .= qq
		    (*** The previous line contains the naughty word "$1".\n)
			       if /\b(tcl|java|python)\b/i;	 # :-)
		   $baz = "\n"; 	       # a one-character string

       qr/STRING/imosx
	       This operator quotes (and possibly compiles) its STRING as a
	       regular expression.  STRING is interpolated the same way as
	       PATTERN in "m/PATTERN/".  If "'" is used as the delimiter, no
	       interpolation is done.  Returns a Perl value which may be used
	       instead of the corresponding "/STRING/imosx" expression.

	       For example,

		   $rex = qr/my.STRING/is;
		   s/$rex/foo/;

	       is equivalent to

		   s/my.STRING/foo/is;

	       The result may be used as a subpattern in a match:

		   $re = qr/$pattern/;
		   $string =~ /foo${re}bar/;   # can be interpolated in other patterns
		   $string =~ $re;	       # or used standalone
		   $string =~ /$re/;	       # or this way

	       Since Perl may compile the pattern at the moment of execution
	       of qr() operator, using qr() may have speed advantages in some
	       situations, notably if the result of qr() is used standalone:

		   sub match {
		       my $patterns = shift;
		       my @compiled = map qr/$_/i, @$patterns;
		       grep {
			   my $success = 0;
			   foreach my $pat (@compiled) {
			       $success = 1, last if /$pat/;
			   }
			   $success;
		       } @_;
		   }

	       Precompilation of the pattern into an internal representation
	       at the moment of qr() avoids a need to recompile the pattern
	       every time a match "/$pat/" is attempted.  (Perl has many other
	       internal optimizations, but none would be triggered in the
	       above example if we did not use qr() operator.)

	       Options are:

		   i   Do case-insensitive pattern matching.
		   m   Treat string as multiple lines.
		   o   Compile pattern only once.
		   s   Treat string as single line.
		   x   Use extended regular expressions.

	       See perlre for additional information on valid syntax for
	       STRING, and for a detailed look at the semantics of regular
	       expressions.

       qx/STRING/
       `STRING`
	       A string which is (possibly) interpolated and then executed as
	       a system command with "/bin/sh" or its equivalent.  Shell wild-
	       cards, pipes, and redirections will be honored.	The collected
	       standard output of the command is returned; standard error is
	       unaffected.  In scalar context, it comes back as a single
	       (potentially multi-line) string, or undef if the command
	       failed.	In list context, returns a list of lines (however
	       you've defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an
	       empty list if the command failed.

	       Because backticks do not affect standard error, use shell file
	       descriptor syntax (assuming the shell supports this) if you
	       care to address this.  To capture a command's STDERR and STDOUT
	       together:

		   $output = `cmd 2>&1`;

	       To capture a command's STDOUT but discard its STDERR:

		   $output = `cmd 2>/dev/null`;

	       To capture a command's STDERR but discard its STDOUT (ordering
	       is important here):

		   $output = `cmd 2>&1 1>/dev/null`;

	       To exchange a command's STDOUT and STDERR in order to capture
	       the STDERR but leave its STDOUT to come out the old STDERR:

		   $output = `cmd 3>&1 1>&2 2>&3 3>&-`;

	       To read both a command's STDOUT and its STDERR separately, it's
	       easiest to redirect them separately to files, and then read
	       from those files when the program is done:

		   system("program args 1>program.stdout 2>program.stderr");

	       Using single-quote as a delimiter protects the command from
	       Perl's double-quote interpolation, passing it on to the shell
	       instead:

		   $perl_info  = qx(ps $$);	       # that's Perl's $$
		   $shell_info = qx'ps $$';	       # that's the new shell's $$

	       How that string gets evaluated is entirely subject to the com-
	       mand interpreter on your system.  On most platforms, you will
	       have to protect shell metacharacters if you want them treated
	       literally.  This is in practice difficult to do, as it's
	       unclear how to escape which characters.	See perlsec for a
	       clean and safe example of a manual fork() and exec() to emulate
	       backticks safely.

	       On some platforms (notably DOS-like ones), the shell may not be
	       capable of dealing with multiline commands, so putting newlines
	       in the string may not get you what you want.  You may be able
	       to evaluate multiple commands in a single line by separating
	       them with the command separator character, if your shell sup-
	       ports that (e.g. ";" on many Unix shells; "&" on the Windows NT
	       "cmd" shell).

	       Beginning with v5.6.0, Perl will attempt to flush all files
	       opened for output before starting the child process, but this
	       may not be supported on some platforms (see perlport).  To be
	       safe, you may need to set $| ($AUTOFLUSH in English) or call
	       the "autoflush()" method of "IO::Handle" on any open handles.

	       Beware that some command shells may place restrictions on the
	       length of the command line.  You must ensure your strings don't
	       exceed this limit after any necessary interpolations.  See the
	       platform-specific release notes for more details about your
	       particular environment.

	       Using this operator can lead to programs that are difficult to
	       port, because the shell commands called vary between systems,
	       and may in fact not be present at all.  As one example, the
	       "type" command under the POSIX shell is very different from the
	       "type" command under DOS.  That doesn't mean you should go out
	       of your way to avoid backticks when they're the right way to
	       get something done.  Perl was made to be a glue language, and
	       one of the things it glues together is commands.  Just under-
	       stand what you're getting yourself into.

	       See "I/O Operators" for more discussion.

       qw/STRING/
	       Evaluates to a list of the words extracted out of STRING, using
	       embedded whitespace as the word delimiters.  It can be under-
	       stood as being roughly equivalent to:

		   split(' ', q/STRING/);

	       the differences being that it generates a real list at compile
	       time, and in scalar context it returns the last element in the
	       list.  So this expression:

		   qw(foo bar baz)

	       is semantically equivalent to the list:

		   'foo', 'bar', 'baz'

	       Some frequently seen examples:

		   use POSIX qw( setlocale localeconv )
		   @EXPORT = qw( foo bar baz );

	       A common mistake is to try to separate the words with comma or
	       to put comments into a multi-line "qw"-string.  For this rea-
	       son, the "use warnings" pragma and the -w switch (that is, the
	       $^W variable) produces warnings if the STRING contains the ","
	       or the "#" character.

       s/PATTERN/REPLACEMENT/egimosx
	       Searches a string for a pattern, and if found, replaces that
	       pattern with the replacement text and returns the number of
	       substitutions made.  Otherwise it returns false (specifically,
	       the empty string).

	       If no string is specified via the "=~" or "!~" operator, the $_
	       variable is searched and modified.  (The string specified with
	       "=~" must be scalar variable, an array element, a hash element,
	       or an assignment to one of those, i.e., an lvalue.)

	       If the delimiter chosen is a single quote, no interpolation is
	       done on either the PATTERN or the REPLACEMENT.  Otherwise, if
	       the PATTERN contains a $ that looks like a variable rather than
	       an end-of-string test, the variable will be interpolated into
	       the pattern at run-time.  If you want the pattern compiled only
	       once the first time the variable is interpolated, use the "/o"
	       option.	If the pattern evaluates to the empty string, the last
	       successfully executed regular expression is used instead.  See
	       perlre for further explanation on these.  See perllocale for
	       discussion of additional considerations that apply when "use
	       locale" is in effect.

	       Options are:

		   e   Evaluate the right side as an expression.
		   g   Replace globally, i.e., all occurrences.
		   i   Do case-insensitive pattern matching.
		   m   Treat string as multiple lines.
		   o   Compile pattern only once.
		   s   Treat string as single line.
		   x   Use extended regular expressions.

	       Any non-alphanumeric, non-whitespace delimiter may replace the
	       slashes.  If single quotes are used, no interpretation is done
	       on the replacement string (the "/e" modifier overrides this,
	       however).  Unlike Perl 4, Perl 5 treats backticks as normal
	       delimiters; the replacement text is not evaluated as a command.
	       If the PATTERN is delimited by bracketing quotes, the REPLACE-
	       MENT has its own pair of quotes, which may or may not be brack-
	       eting quotes, e.g., "s(foo)(bar)" or "s/bar/".  A "/e"
	       will cause the replacement portion to be treated as a full-
	       fledged Perl expression and evaluated right then and there.  It
	       is, however, syntax checked at compile-time. A second "e" modi-
	       fier will cause the replacement portion to be "eval"ed before
	       being run as a Perl expression.

	       Examples:

		   s/\bgreen\b/mauve/g; 	       # don't change wintergreen

		   $path =~ s|/usr/bin|/usr/local/bin|;

		   s/Login: $foo/Login: $bar/; # run-time pattern

		   ($foo = $bar) =~ s/this/that/;      # copy first, then change

		   $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

		   $_ = 'abc123xyz';
		   s/\d+/$&*2/e;	       # yields 'abc246xyz'
		   s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
		   s/\w/$& x 2/eg;	       # yields 'aabbcc  224466xxyyzz'

		   s/%(.)/$percent{$1}/g;      # change percent escapes; no /e
		   s/%(.)/$percent{$1} || $&/ge;       # expr now, so /e
		   s/^=(\w+)/&pod($1)/ge;      # use function call

		   # expand variables in $_, but dynamics only, using
		   # symbolic dereferencing
		   s/\$(\w+)/${$1}/g;

		   # Add one to the value of any numbers in the string
		   s/(\d+)/1 + $1/eg;

		   # This will expand any embedded scalar variable
		   # (including lexicals) in $_ : First $1 is interpolated
		   # to the variable name, and then evaluated
		   s/(\$\w+)/$1/eeg;

		   # Delete (most) C comments.
		   $program =~ s {
		       /\*     # Match the opening delimiter.
		       .*?     # Match a minimal number of characters.
		       \*/     # Match the closing delimiter.
		   } []gsx;

		   s/^\s*(.*?)\s*$/$1/;        # trim whitespace in $_, expensively

		   for ($variable) {	       # trim whitespace in $variable, cheap
		       s/^\s+//;
		       s/\s+$//;
		   }

		   s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields

	       Note the use of $ instead of \ in the last example.  Unlike
	       sed, we use the \<digit> form in only the left hand side.  Any-
	       where else it's $<digit>.

	       Occasionally, you can't use just a "/g" to get all the changes
	       to occur that you might want.  Here are two common cases:

		   # put commas in the right places in an integer
		   1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

		   # expand tabs to 8-column spacing
		   1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

       tr/SEARCHLIST/REPLACEMENTLIST/cds
       y/SEARCHLIST/REPLACEMENTLIST/cds
	       Transliterates all occurrences of the characters found in the
	       search list with the corresponding character in the replacement
	       list.  It returns the number of characters replaced or deleted.
	       If no string is specified via the =~ or !~ operator, the $_
	       string is transliterated.  (The string specified with =~ must
	       be a scalar variable, an array element, a hash element, or an
	       assignment to one of those, i.e., an lvalue.)

	       A character range may be specified with a hyphen, so
	       "tr/A-J/0-9/" does the same replacement as "tr/ACEG-
	       IBDFHJ/0246813579/".  For sed devotees, "y" is provided as a
	       synonym for "tr".  If the SEARCHLIST is delimited by bracketing
	       quotes, the REPLACEMENTLIST has its own pair of quotes, which
	       may or may not be bracketing quotes, e.g., "tr[A-Z][a-z]" or
	       "tr(+\-*/)/ABCD/".

	       Note that "tr" does not do regular expression character classes
	       such as "\d" or "[:lower:]".  The 
=29473
+350
(49)