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PERLRE(1) Perl Programmers Reference Guide PERLRE(1)
NAME
perlre - Perl regular expressions
DESCRIPTION
This page describes the syntax of regular expressions in Perl.
If you haven't used regular expressions before, a quick-start introduc-
tion is available in perlrequick, and a longer tutorial introduction is
available in perlretut.
For reference on how regular expressions are used in matching opera-
tions, plus various examples of the same, see discussions of "m//",
"s///", "qr//" and "??" in "Regexp Quote-Like Operators" in perlop.
Matching operations can have various modifiers. Modifiers that relate
to the interpretation of the regular expression inside are listed
below. Modifiers that alter the way a regular expression is used by
Perl are detailed in "Regexp Quote-Like Operators" in perlop and "Gory
details of parsing quoted constructs" in perlop.
i Do case-insensitive pattern matching.
If "use locale" is in effect, the case map is taken from the cur-
rent locale. See perllocale.
m Treat string as multiple lines. That is, change "^" and "$" from
matching the start or end of the string to matching the start or
end of any line anywhere within the string.
s Treat string as single line. That is, change "." to match any
character whatsoever, even a newline, which normally it would not
match.
The "/s" and "/m" modifiers both override the $* setting. That is,
no matter what $* contains, "/s" without "/m" will force "^" to
match only at the beginning of the string and "$" to match only at
the end (or just before a newline at the end) of the string.
Together, as /ms, they let the "." match any character whatsoever,
while still allowing "^" and "$" to match, respectively, just after
and just before newlines within the string.
x Extend your pattern's legibility by permitting whitespace and com-
ments.
These are usually written as "the "/x" modifier", even though the
delimiter in question might not really be a slash. Any of these modi-
fiers may also be embedded within the regular expression itself using
the "(?...)" construct. See below.
The "/x" modifier itself needs a little more explanation. It tells the
regular expression parser to ignore whitespace that is neither back-
slashed nor within a character class. You can use this to break up
your regular expression into (slightly) more readable parts. The "#"
character is also treated as a metacharacter introducing a comment,
just as in ordinary Perl code. This also means that if you want real
whitespace or "#" characters in the pattern (outside a character class,
where they are unaffected by "/x"), that you'll either have to escape
them or encode them using octal or hex escapes. Taken together, these
features go a long way towards making Perl's regular expressions more
readable. Note that you have to be careful not to include the pattern
delimiter in the comment--perl has no way of knowing you did not intend
to close the pattern early. See the C-comment deletion code in perlop.
Regular Expressions
The patterns used in Perl pattern matching derive from supplied in the
Version 8 regex routines. (The routines are derived (distantly) from
Henry Spencer's freely redistributable reimplementation of the V8 rou-
tines.) See "Version 8 Regular Expressions" for details.
In particular the following metacharacters have their standard
egrep-ish meanings:
\ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
| Alternation
() Grouping
[] Character class
By default, the "^" character is guaranteed to match only the beginning
of the string, the "$" character only the end (or before the newline at
the end), and Perl does certain optimizations with the assumption that
the string contains only one line. Embedded newlines will not be
matched by "^" or "$". You may, however, wish to treat a string as a
multi-line buffer, such that the "^" will match after any newline
within the string, and "$" will match before any newline. At the cost
of a little more overhead, you can do this by using the /m modifier on
the pattern match operator. (Older programs did this by setting $*,
but this practice is now deprecated.)
To simplify multi-line substitutions, the "." character never matches a
newline unless you use the "/s" modifier, which in effect tells Perl to
pretend the string is a single line--even if it isn't. The "/s" modi-
fier also overrides the setting of $*, in case you have some (badly
behaved) older code that sets it in another module.
The following standard quantifiers are recognized:
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
(If a curly bracket occurs in any other context, it is treated as a
regular character. In particular, the lower bound is not optional.)
The "*" modifier is equivalent to "{0,}", the "+" modifier to "{1,}",
and the "?" modifier to "{0,1}". n and m are limited to integral val-
ues less than a preset limit defined when perl is built. This is usu-
ally 32766 on the most common platforms. The actual limit can be seen
in the error message generated by code such as this:
$_ **= $_ , / {$_} / for 2 .. 42;
By default, a quantified subpattern is "greedy", that is, it will match
as many times as possible (given a particular starting location) while
still allowing the rest of the pattern to match. If you want it to
match the minimum number of times possible, follow the quantifier with
a "?". Note that the meanings don't change, just the "greediness":
*? Match 0 or more times
+? Match 1 or more times
?? Match 0 or 1 time
{n}? Match exactly n times
{n,}? Match at least n times
{n,m}? Match at least n but not more than m times
Because patterns are processed as double quoted strings, the following
also work:
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\033 octal char (think of a PDP-11)
\x1B hex char
\x{263a} wide hex char (Unicode SMILEY)
\c[ control char
\N{name} named char
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase till \E (think vi)
\U uppercase till \E (think vi)
\E end case modification (think vi)
\Q quote (disable) pattern metacharacters 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. For documenta-
tion of "\N{name}", see charnames.
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 matched. You'll need
to write something like "m/\Quser\E\@\Qhost/".
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
\W Match a non-"word" character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a digit character
\D Match a non-digit character
\pP Match P, named property. Use \p{Prop} for longer names.
\PP Match non-P
\X Match eXtended Unicode "combining character sequence",
equivalent to (?:\PM\pM*)
\C Match a single C char (octet) even under Unicode.
NOTE: breaks up characters into their UTF-8 bytes,
so you may end up with malformed pieces of UTF-8.
Unsupported in lookbehind.
A "\w" matches a single alphanumeric character (an alphabetic charac-
ter, or a decimal digit) or "_", not a whole word. Use "\w+" to match
a string of Perl-identifier characters (which isn't the same as match-
ing an English word). If "use locale" is in effect, the list of alpha-
betic characters generated by "\w" is taken from the current locale.
See perllocale. You may use "\w", "\W", "\s", "\S", "\d", and "\D"
within character classes, but if you try to use them as endpoints of a
range, that's not a range, the "-" is understood literally. If Unicode
is in effect, "\s" matches also "\x{85}", "\x{2028}, and "\x{2029}",
see perlunicode for more details about "\pP", "\PP", and "\X", and per-
luniintro about Unicode in general. You can define your own "\p" and
"\P" properties, see perlunicode.
The POSIX character class syntax
[:class:]
is also available. The available classes and their backslash equiva-
lents (if available) are as follows:
alpha
alnum
ascii
blank [1]
cntrl
digit \d
graph
lower
print
punct
space \s [2]
upper
word \w [3]
xdigit
[1] A GNU extension equivalent to "[ \t]", "all horizontal whitespace".
[2] Not exactly equivalent to "\s" since the "[[:space:]]" includes
also the (very rare) "vertical tabulator", "\ck", chr(11).
[3] A Perl extension, see above.
For example use "[:upper:]" to match all the uppercase characters.
Note that the "[]" are part of the "[::]" construct, not part of the
whole character class. For example:
[01[:alpha:]%]
matches zero, one, any alphabetic character, and the percentage sign.
The following equivalences to Unicode \p{} constructs and equivalent
backslash character classes (if available), will hold:
[:...:] \p{...} backslash
alpha IsAlpha
alnum IsAlnum
ascii IsASCII
blank IsSpace
cntrl IsCntrl
digit IsDigit \d
graph IsGraph
lower IsLower
print IsPrint
punct IsPunct
space IsSpace
IsSpacePerl \s
upper IsUpper
word IsWord
xdigit IsXDigit
For example "[:lower:]" and "\p{IsLower}" are equivalent.
If the "utf8" pragma is not used but the "locale" pragma is, the
classes correlate with the usual isalpha(3) interface (except for
"word" and "blank").
The assumedly non-obviously named classes are:
cntrl
Any control character. Usually characters that don't produce out-
put as such but instead control the terminal somehow: for example
newline and backspace are control characters. All characters with
ord() less than 32 are most often classified as control characters
(assuming ASCII, the ISO Latin character sets, and Unicode), as is
the character with the ord() value of 127 ("DEL").
graph
Any alphanumeric or punctuation (special) character.
print
Any alphanumeric or punctuation (special) character or the space
character.
punct
Any punctuation (special) character.
xdigit
Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f]
would work just fine) it is included for completeness.
You can negate the [::] character classes by prefixing the class name
with a '^'. This is a Perl extension. For example:
POSIX traditional Unicode
[:^digit:] \D \P{IsDigit}
[:^space:] \S \P{IsSpace}
[:^word:] \W \P{IsWord}
Perl respects the POSIX standard in that POSIX character classes are
only supported within a character class. The POSIX character classes
[.cc.] and [=cc=] are recognized but not supported and trying to use
them will cause an error.
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match a non-(word boundary)
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
A word boundary ("\b") is a spot between two characters that has a "\w"
on one side of it and a "\W" on the other side of it (in either order),
counting the imaginary characters off the beginning and end of the
string as matching a "\W". (Within character classes "\b" represents
backspace rather than a word boundary, just as it normally does in any
double-quoted string.) The "\A" and "\Z" are just like "^" and "$",
except that they won't match multiple times when the "/m" modifier is
used, while "^" and "$" will match at every internal line boundary. To
match the actual end of the string and not ignore an optional trailing
newline, use "\z".
The "\G" assertion can be used to chain global matches (using "m//g"),
as described in "Regexp Quote-Like Operators" in perlop. It is also
useful when writing "lex"-like scanners, when you have several patterns
that you want to match against consequent substrings of your string,
see the previous reference. The actual location where "\G" will match
can also be influenced by using "pos()" as an lvalue: see "pos" in
perlfunc. Currently "\G" is only fully supported when anchored to the
start of the pattern; while it is permitted to use it elsewhere, as in
"/(?<=\G..)./g", some such uses ("/.\G/g", for example) currently cause
problems, and it is recommended that you avoid such usage for now.
The bracketing construct "( ... )" creates capture buffers. To refer
to the digit'th buffer use \ within the match. Outside the
match use "$" instead of "\". (The \ notation works in certain
circumstances outside the match. See the warning below about \1 vs $1
for details.) Referring back to another part of the match is called a
backreference.
There is no limit to the number of captured substrings that you may
use. However Perl also uses \10, \11, etc. as aliases for \010, \011,
etc. (Recall that 0 means octal, so \011 is the character at number 9
in your coded character set; which would be the 10th character, a hori-
zontal tab under ASCII.) Perl resolves this ambiguity by interpreting
\10 as a backreference only if at least 10 left parentheses have opened
before it. Likewise \11 is a backreference only if at least 11 left
parentheses have opened before it. And so on. \1 through \9 are
always interpreted as backreferences.
Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/(.)\1/) { # find first doubled char
print "'$1' is the first doubled character\n";
}
if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
Several special variables also refer back to portions of the previous
match. $+ returns whatever the last bracket match matched. $& returns
the entire matched string. (At one point $0 did also, but now it
returns the name of the program.) $` returns everything before the
matched string. $' returns everything after the matched string. And
$^N contains whatever was matched by the most-recently closed group
(submatch). $^N can be used in extended patterns (see below), for exam-
ple to assign a submatch to a variable.
The numbered match variables ($1, $2, $3, etc.) and the related punctu-
ation set ($+, $&, $`, $', and $^N) are all dynamically scoped until
the end of the enclosing block or until the next successful match,
whichever comes first. (See "Compound Statements" in perlsyn.)
NOTE: failed matches in Perl do not reset the match variables, which
makes it easier to write code that tests for a series of more specific
cases and remembers the best match.
WARNING: Once Perl sees that you need one of $&, $`, or $' anywhere in
the program, it has to provide them for every pattern match. This may
substantially slow your program. Perl uses the same mechanism to pro-
duce $1, $2, etc, so you also pay a price for each pattern that con-
tains capturing parentheses. (To avoid this cost while retaining the
grouping behaviour, use the extended regular expression "(?: ... )"
instead.) But if you never use $&, $` or $', then patterns without
capturing parentheses will not be penalized. So avoid $&, $', and $`
if you can, but if you can't (and some algorithms really appreciate
them), once you've used them once, use them at will, because you've
already paid the price. As of 5.005, $& is not so costly as the other
two.
Backslashed metacharacters in Perl are alphanumeric, such as "\b",
"\w", "\n". Unlike some other regular expression languages, there are
no backslashed symbols that aren't alphanumeric. So anything that
looks like \\, \(, \), \<, \>, \{, or \} is always interpreted as a
literal character, not a metacharacter. This was once used in a common
idiom to disable or quote the special meanings of regular expression
metacharacters in a string that you want to use for a pattern. Simply
quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
(If "use locale" is set, then this depends on the current locale.)
Today it is more common to use the quotemeta() function or the "\Q"
metaquoting escape sequence to disable all metacharacters' special
meanings like this:
/$unquoted\Q$quoted\E$unquoted/
Beware that if you put literal backslashes (those not inside interpo-
lated variables) between "\Q" and "\E", double-quotish backslash inter-
polation may lead to confusing results. If you need to use literal
backslashes within "\Q...\E", consult "Gory details of parsing quoted
constructs" in perlop.
Extended Patterns
Perl also defines a consistent extension syntax for features not found
in standard tools like awk and lex. The syntax is a pair of parenthe-
ses with a question mark as the first thing within the parentheses.
The character after the question mark indicates the extension.
The stability of these extensions varies widely. Some have been part
of the core language for many years. Others are experimental and may
change without warning or be completely removed. Check the documenta-
tion on an individual feature to verify its current status.
A question mark was chosen for this and for the minimal-matching con-
struct because 1) question marks are rare in older regular expressions,
and 2) whenever you see one, you should stop and "question" exactly
what is going on. That's psychology...
"(?#text)"
A comment. The text is ignored. If the "/x" modifier
enables whitespace formatting, a simple "#" will suffice.
Note that Perl closes the comment as soon as it sees a ")",
so there is no way to put a literal ")" in the comment.
"(?imsx-imsx)"
One or more embedded pattern-match modifiers, to be turned on
(or turned off, if preceded by "-") for the remainder of the
pattern or the remainder of the enclosing pattern group (if
any). This is particularly useful for dynamic patterns, such
as those read in from a configuration file, read in as an
argument, are specified in a table somewhere, etc. Consider
the case that some of which want to be case sensitive and
some do not. The case insensitive ones need to include
merely "(?i)" at the front of the pattern. For example:
$pattern = "foobar";
if ( /$pattern/i ) { }
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ ) { }
These modifiers are restored at the end of the enclosing
group. For example,
( (?i) blah ) \s+ \1
will match a repeated (including the case!) word "blah" in
any case, assuming "x" modifier, and no "i" modifier outside
this group.
"(?:pattern)"
"(?imsx-imsx:pattern)"
This is for clustering, not capturing; it groups subexpres-
sions like "()", but doesn't make backreferences as "()"
does. So
@fields = split(/\b(?:a|b|c)\b/)
is like
@fields = split(/\b(a|b|c)\b/)
but doesn't spit out extra fields. It's also cheaper not to
capture characters if you don't need to.
Any letters between "?" and ":" act as flags modifiers as
with "(?imsx-imsx)". For example,
/(?s-i:more.*than).*million/i
is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
"(?=pattern)"
A zero-width positive look-ahead assertion. For example,
"/\w+(?=\t)/" matches a word followed by a tab, without
including the tab in $&.
"(?!pattern)"
A zero-width negative look-ahead assertion. For example
"/foo(?!bar)/" matches any occurrence of "foo" that isn't
followed by "bar". Note however that look-ahead and look-
behind are NOT the same thing. You cannot use this for
look-behind.
If you are looking for a "bar" that isn't preceded by a
"foo", "/(?!foo)bar/" will not do what you want. That's
because the "(?!foo)" is just saying that the next thing can-
not be "foo"--and it's not, it's a "bar", so "foobar" will
match. You would have to do something like "/(?!foo)...bar/"
for that. We say "like" because there's the case of your
"bar" not having three characters before it. You could cover
that this way: "/(?:(?!foo)...|^.{0,2})bar/". Sometimes it's
still easier just to say:
if (/bar/ && $` !~ /foo$/)
For look-behind see below.
"(?<=pattern)"
A zero-width positive look-behind assertion. For example,
"/(?<=\t)\w+/" matches a word that follows a tab, without
including the tab in $&. Works only for fixed-width
look-behind.
"(?WARNING: This extended regular expression feature is consid-
ered highly experimental, and may be changed or deleted with-
out notice.
This zero-width assertion evaluates any embedded Perl code.
It always succeeds, and its "code" is not interpolated. Cur-
rently, the rules to determine where the "code" ends are
somewhat convoluted.
This feature can be used together with the special variable
$^N to capture the results of submatches in variables without
having to keep track of the number of nested parentheses. For
example:
$_ = "The brown fox jumps over the lazy dog";
/the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
print "color = $color, animal = $animal\n";
Inside the "(?{...})" block, $_ refers to the string the reg-
ular expression is matching against. You can also use "pos()"
to know what is the current position of matching within this
string.
The "code" is properly scoped in the following sense: If the
assertion is backtracked (compare "Backtracking"), all
changes introduced after "local"ization are undone, so that
$_ = 'a' x 8;
m<
(?{ $cnt = 0 }) # Initialize $cnt.
(
a
(?{
local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
})
)*
aaaa
(?{ $res = $cnt }) # On success copy to non-localized
# location.
>x;
will set "$res = 4". Note that after the match, $cnt returns
to the globally introduced value, because the scopes that
restrict "local" operators are unwound.
This assertion may be used as a "(?(condition)yes-pat-
tern|no-pattern)" switch. If not used in this way, the
result of evaluation of "code" is put into the special vari-
able $^R. This happens immediately, so $^R can be used from
other "(?{ code })" assertions inside the same regular
expression.
The assignment to $^R above is properly localized, so the old
value of $^R is restored if the assertion is backtracked;
compare "Backtracking".
For reasons of security, this construct is forbidden if the
regular expression involves run-time interpolation of vari-
ables, unless the perilous "use re 'eval'" pragma has been
used (see re), or the variables contain results of "qr//"
operator (see "qr/STRING/imosx" in perlop).
This restriction is because of the wide-spread and remarkably
convenient custom of using run-time determined strings as
patterns. For example:
$re = <>;
chomp $re;
$string =~ /$re/;
Before Perl knew how to execute interpolated code within a
pattern, this operation was completely safe from a security
point of view, although it could raise an exception from an
illegal pattern. If you turn on the "use re 'eval'", though,
it is no longer secure, so you should only do so if you are
also using taint checking. Better yet, use the carefully
constrained evaluation within a Safe compartment. See
perlsec for details about both these mechanisms.
"(??{ code })"
WARNING: This extended regular expression feature is consid-
ered highly experimental, and may be changed or deleted with-
out notice. A simplified version of the syntax may be intro-
duced for commonly used idioms.
This is a "postponed" regular subexpression. The "code" is
evaluated at run time, at the moment this subexpression may
match. The result of evaluation is considered as a regular
expression and matched as if it were inserted instead of this
construct.
The "code" is not interpolated. As before, the rules to
determine where the "code" ends are currently somewhat convo-
luted.
The following pattern matches a parenthesized group:
$re = qr{
\(
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(??{ $re }) # Group with matching parens
)*
\)
}x;
"(?>pattern)"
WARNING: This extended regular expression feature is consid-
ered highly experimental, and may be changed or deleted with-
out notice.
An "independent" subexpression, one which matches the sub-
string that a standalone "pattern" would match if anchored at
the given position, and it matches nothing other than this
substring. This construct is useful for optimizations of
what would otherwise be "eternal" matches, because it will
not backtrack (see "Backtracking"). It may also be useful in
places where the "grab all you can, and do not give anything
back" semantic is desirable.
For example: "^(?>a*)ab" will never match, since "(?>a*)"
(anchored at the beginning of string, as above) will match
all characters "a" at the beginning of string, leaving no "a"
for "ab" to match. In contrast, "a*ab" will match the same
as "a+b", since the match of the subgroup "a*" is influenced
by the following group "ab" (see "Backtracking"). In partic-
ular, "a*" inside "a*ab" will match fewer characters than a
standalone "a*", since this makes the tail match.
An effect similar to "(?>pattern)" may be achieved by writing
"(?=(pattern))\1". This matches the same substring as a
standalone "a+", and the following "\1" eats the matched
string; it therefore makes a zero-length assertion into an
analogue of "(?>...)". (The difference between these two
constructs is that the second one uses a capturing group,
thus shifting ordinals of backreferences in the rest of a
regular expression.)
Consider this pattern:
m{ \(
(
[^()]+ # x+
|
\( [^()]* \)
)+
\)
}x
That will efficiently match a nonempty group with matching
parentheses two levels deep or less. However, if there is no
such group, it will take virtually forever on a long string.
That's because there are so many different ways to split a
long string into several substrings. This is what "(.+)+" is
doing, and "(.+)+" is similar to a subpattern of the above
pattern. Consider how the pattern above detects no-match on
"((()aaaaaaaaaaaaaaaaaa" in several seconds, but that each
extra letter doubles this time. This exponential performance
will make it appear that your program has hung. However, a
tiny change to this pattern
m{ \(
(
(?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
\)
}x
which uses "(?>...)" matches exactly when the one above does
(verifying this yourself would be a productive exercise), but
finishes in a fourth the time when used on a similar string
with 1000000 "a"s. Be aware, however, that this pattern cur-
rently triggers a warning message under the "use warnings"
pragma or -w switch saying it "matches null string many times
in regex".
On simple groups, such as the pattern "(?> [^()]+ )", a com-
parable effect may be achieved by negative look-ahead, as in
"[^()]+ (?! [^()] )". This was only 4 times slower on a
string with 1000000 "a"s.
The "grab all you can, and do not give anything back" seman-
tic is desirable in many situations where on the first sight
a simple "()*" looks like the correct solution. Suppose we
parse text with comments being delimited by "#" followed by
some optional (horizontal) whitespace. Contrary to its
appearance, "#[ \t]*" is not the correct subexpression to
match the comment delimiter, because it may "give up" some
whitespace if the remainder of the pattern can be made to
match that way. The correct answer is either one of these:
(?>#[ \t]*)
#[ \t]*(?![ \t])
For example, to grab non-empty comments into $1, one should
use either one of these:
/ (?> \# [ \t]* ) ( .+ ) /x;
/ \# [ \t]* ( [^ \t] .* ) /x;
Which one you pick depends on which of these expressions bet-
ter reflects the above specification of comments.
"(?(condition)yes-pattern|no-pattern)"
"(?(condition)yes-pattern)"
WARNING: This extended regular expression feature is consid-
ered highly experimental, and may be changed or deleted with-
out notice.
Conditional expression. "(condition)" should be either an
integer in parentheses (which is valid if the corresponding
pair of parentheses matched), or look-ahead/look-behind/eval-
uate zero-width assertion.
For example:
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly included in
parentheses themselves.
Backtracking
NOTE: This section presents an abstract approximation of regular
expression behavior. For a more rigorous (and complicated) view of the
rules involved in selecting a match among possible alternatives, see
"Combining pieces together".
A fundamental feature of regular expression matching involves the
notion called backtracking, which is currently used (when needed) by
all regular expression quantifiers, namely "*", "*?", "+", "+?",
"{n,m}", and "{n,m}?". Backtracking is often optimized internally, but
the general principle outlined here is valid.
For a regular expression to match, the entire regular expression must
match, not just part of it. So if the beginning of a pattern contain-
ing a quantifier succeeds in a way that causes later parts in the pat-
tern to fail, the matching engine backs up and recalculates the begin-
ning part--that's why it's called backtracking.
Here is an example of backtracking: Let's say you want to find the
word following "foo" in the string "Food is on the foo table.":
$_ = "Food is on the foo table.";
if ( /\b(foo)\s+(\w+)/i ) {
print "$2 follows $1.\n";
}
When the match runs, the first part of the regular expression
("\b(foo)") finds a possible match right at the beginning of the
string, and loads up $1 with "Foo". However, as soon as the matching
engine sees that there's no whitespace following the "Foo" that it had
saved in $1, it realizes its mistake and starts over again one charac-
ter after where it had the tentative match. This time it goes all the
way until the next occurrence of "foo". The complete regular expression
matches this time, and you get the expected output of "table follows
foo."
Sometimes minimal matching can help a lot. Imagine you'd like to match
everything between "foo" and "bar". Initially, you write something
like this:
$_ = "The food is under the bar in the barn.";
if ( /foo(.*)bar/ ) {
print "got <$1>\n";
}
Which perhaps unexpectedly yields:
got
That's because ".*" was greedy, so you get everything between the first
"foo" and the last "bar". Here it's more effective to use minimal
matching to make sure you get the text between a "foo" and the first
"bar" thereafter.
if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
got
Here's another example: let's say you'd like to match a number at the
end of a string, and you also want to keep the preceding part of the
match. So you write this:
$_ = "I have 2 numbers: 53147";
if ( /(.*)(\d*)/ ) { # Wrong!
print "Beginning is <$1>, number is <$2>.\n";
}
That won't work at all, because ".*" was greedy and gobbled up the
whole string. As "\d*" can match on an empty string the complete regu-
lar expression matched successfully.
Beginning is , number is <>.
Here are some variants, most of which don't work:
$_ = "I have 2 numbers: 53147";
@pats = qw{
(.*)(\d*)
(.*)(\d+)
(.*?)(\d*)
(.*?)(\d+)
(.*)(\d+)$
(.*?)(\d+)$
(.*)\b(\d+)$
(.*\D)(\d+)$
};
for $pat (@pats) {
printf "%-12s ", $pat;
if ( /$pat/ ) {
print "<$1> <$2>\n";
} else {
print "FAIL\n";
}
}
That will print out:
(.*)(\d*) <>
(.*)(\d+) <7>
(.*?)(\d*) <> <>
(.*?)(\d+) <2>
(.*)(\d+)$ <7>
(.*?)(\d+)$ <53147>
(.*)\b(\d+)$ <53147>
(.*\D)(\d+)$ <53147>
As you see, this can be a bit tricky. It's important to realize that a
regular expression is merely a set of assertions that gives a defini-
tion of success. There may be 0, 1, or several different ways that the
definition might succeed against a particular string. And if there are
multiple ways it might succeed, you need to understand backtracking to
know which variety of success you will achieve.
When using look-ahead assertions and negations, this can all get even
trickier. Imagine you'd like to find a sequence of non-digits not fol-
lowed by "123". You might try to write that as
$_ = "ABC123";
if ( /^\D*(?!123)/ ) { # Wrong!
print "Yup, no 123 in $_\n";
}
But that isn't going to match; at least, not the way you're hoping. It
claims that there is no 123 in the string. Here's a clearer picture of
why that pattern matches, contrary to popular expectations:
$x = 'ABC123';
$y = 'ABC445';
print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
This prints
2: got ABC
3: got AB
4: got ABC
You might have expected test 3 to fail because it seems to a more gen-
eral purpose version of test 1. The important difference between them
is that test 3 contains a quantifier ("\D*") and so can use backtrack-
ing, whereas test 1 will not. What's happening is that you've asked
"Is it true that at the start of $x, following 0 or more non-digits,
you have something that's not 123?" If the pattern matcher had let
"\D*" expand to "ABC", this would have caused the whole pattern to
fail.
The search engine will initially match "\D*" with "ABC". Then it will
try to match "(?!123" with "123", which fails. But because a quanti-
fier ("\D*") has been used in the regular expression, the search engine
can backtrack and retry the match differently in the hope of matching
the complete regular expression.
The pattern really, really wants to succeed, so it uses the standard
pattern back-off-and-retry and lets "\D*" expand to just "AB" this
time. Now there's indeed something following "AB" that is not "123".
It's "C123", which suffices.
We can deal with this by using both an assertion and a negation. We'll
say that the first part in $1 must be followed both by a digit and by
something that's not "123". Remember that the look-aheads are zero-
width expressions--they only look, but don't consume any of the string
in their match. So rewriting this way produces what you'd expect; that
is, case 5 will fail, but case 6 succeeds:
print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
6: got ABC
In other words, the two zero-width assertions next to each other work
as though they're ANDed together, just as you'd use any built-in asser-
tions: "/^$/" matches only if you're at the beginning of the line AND
the end of the line simultaneously. The deeper underlying truth is
that juxtaposition in regular expressions always means AND, except when
you write an explicit OR using the vertical bar. "/ab/" means match
"a" AND (then) match "b", although the attempted matches are made at
different positions because "a" is not a zero-width assertion, but a
one-width assertion.
WARNING: particularly complicated regular expressions can take exponen-
tial time to solve because of the immense number of possible ways they
can use backtracking to try match. For example, without internal opti-
mizations done by the regular expression engine, this will take a
painfully long time to run:
'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
And if you used "*"'s in the internal groups instead of limiting them
to 0 through 5 matches, then it would take forever--or until you ran
out of stack space. Moreover, these internal optimizations are not
always applicable. For example, if you put "{0,5}" instead of "*" on
the external group, no current optimization is applicable, and the
match takes a long time to finish.
A powerful tool for optimizing such beasts is what is known as an
"independent group", which does not backtrack (see ""(?>pattern)"").
Note also that zero-length look-ahead/look-behind assertions will not
backtrack to make the tail match, since they are in "logical" context:
only whether they match is considered relevant. For an example where
side-effects of look-ahead might have influenced the following match,
see ""(?>pattern)"".
Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8 regex rou-
tines, here are the pattern-matching rules not described above.
Any single character matches itself, unless it is a metacharacter with
a special meaning described here or above. You can cause characters
that normally function as metacharacters to be interpreted literally by
prefixing them with a "\" (e.g., "\." matches a ".", not any character;
"\\" matches a "\"). A series of characters matches that series of
characters in the target string, so the pattern "blurfl" would match
"blurfl" in the target string.
You can specify a character class, by enclosing a list of characters in
"[]", which will match any one character from the list. If the first
character after the "[" is "^", the class matches any character not in
the list. Within a list, the "-" character specifies a range, so that
"a-z" represents all characters between "a" and "z", inclusive. If you
want either "-" or "]" itself to be a member of a class, put it at the
start of the list (possibly after a "^"), or escape it with a back-
slash. "-" is also taken literally when it is at the end of the list,
just before the closing "]". (The following all specify the same class
of three characters: "[-az]", "[az-]", and "[a\-z]". All are different
from "[a-z]", which specifies a class containing twenty-six characters,
even on EBCDIC based coded character sets.) Also, if you try to use
the character classes "\w", "\W", "\s", "\S", "\d", or "\D" as end-
points of a range, that's not a range, the "-" is understood literally.
Note also that the whole range idea is rather unportable between char-
acter 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 ([0-9]). Anything else is unsafe. If in doubt, spell out the
character sets in full.
Characters may be specified using a metacharacter syntax much like that
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc. More generally, \nnn, where nnn is a string of
octal digits, matches the character whose coded character set value is
nnn. Similarly, \xnn, where nn are hexadecimal digits, matches the
character whose numeric value is nn. The expression \cx matches the
character control-x. Finally, the "." metacharacter matches any char-
acter except "\n" (unless you use "/s").
You can specify a series of alternatives for a pattern using "|" to
separate them, so that "fee|fie|foe" will match any of "fee", "fie", or
"foe" in the target string (as would "f(e|i|o)e"). The first alterna-
tive includes everything from the last pattern delimiter ("(", "[", or
the beginning of the pattern) up to the first "|", and the last alter-
native contains everything from the last "|" to the next pattern delim-
iter. That's why it's common practice to include alternatives in
parentheses: to minimize confusion about where they start and end.
Alternatives are tried from left to right, so the first alternative
found for which the entire expression matches, is the one that is cho-
sen. This means that alternatives are not necessarily greedy. For exam-
ple: when matching "foo|foot" against "barefoot", only the "foo" part
will match, as that is the first alternative tried, and it successfully
matches the target string. (This might not seem important, but it is
important when you are capturing matched text using parentheses.)
Also remember that "|" is interpreted as a literal within square brack-
ets, so if you write "[fee|fie|foe]" you're really only matching
"[feio|]".
Within a pattern, you may designate subpatterns for later reference by
enclosing them in parentheses, and you may refer back to the nth sub-
pattern later in the pattern using the metacharacter \n. Subpatterns
are numbered based on the left to right order of their opening paren-
thesis. A backreference matches whatever actually matched the subpat-
tern in the string being examined, not the rules for that subpattern.
Therefore, "(0|0x)\d*\s\1\d*" will match "0x1234 0x4321", but not
"0x1234 01234", because subpattern 1 matched "0x", even though the rule
"0|0x" could potentially match the leading 0 in the second number.
Warning on \1 vs $1
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the RHS of a substitute to avoid shocking the
sed addicts, but it's a dirty habit to get into. That's because in
PerlThink, the righthand side of an "s///" is a double-quoted string.
"\1" in the usual double-quoted string means a control-A. The custom-
ary Unix meaning of "\1" is kludged in for "s///". However, if you get
into the habit of doing that, you get yourself into trouble if you then
add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying "\{1}000", whereas you can fix it
with "${1}000". The operation of interpolation should not be confused
with the operation of matching a backreference. Certainly they mean
two different things on the left side of the "s///".
Repeated patterns matching zero-length substring
WARNING: Difficult material (and prose) ahead. This section needs a
rewrite.
Regular expressions provide a terse and powerful programming language.
As with most other power tools, power comes together with the ability
to wreak havoc.
A common abuse of this power stems from the ability to make infinite
loops using regular expressions, with something as innocuous as:
'foo' =~ m{ ( o? )* }x;
The "o?" can match at the beginning of 'foo', and since the position in
the string is not moved by the match, "o?" would match again and again
because of the "*" modifier. Another common way to create a similar
cycle is with the looping modifier "//g":
@matches = ( 'foo' =~ m{ o? }xg );
or
print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
or the loop implied by split().
However, long experience has shown that many programming tasks may be
significantly simplified by using repeated subexpressions that may
match zero-length substrings. Here's a simple example being:
@chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
Thus Perl allows such constructs, by forcefully breaking the infinite
loop. The rules for this are different for lower-level loops given by
the greedy modifiers "*+{}", and for higher-level ones like the "/g"
modifier or split() operator.
The lower-level loops are interrupted (that is, the loop is broken)
when Perl detects that a repeated expression matched a zero-length sub-
string. Thus
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
is made equivalent to
m{ (?: NON_ZERO_LENGTH )*
|
(?: ZERO_LENGTH )?
}x;
The higher level-loops preserve an additional state between iterations:
whether the last match was zero-length. To break the loop, the follow-
ing match after a zero-length match is prohibited to have a length of
zero. This prohibition interacts with backtracking (see "Backtrack-
ing"), and so the second best match is chosen if the best match is of
zero length.
For example:
$_ = 'bar';
s/\w??/<$&>/g;
results in "<><><><>". At each position of the string the
best match given by non-greedy "??" is the zero-length match, and the
second best match is what is matched by "\w". Thus zero-length matches
alternate with one-character-long matches.
Similarly, for repeated "m/()/g" the second-best match is the match at
the position one notch further in the string.
The additional state of being matched with zero-length is associated
with the matched string, and is reset by each assignment to pos().
Zero-length matches at the end of the previous match are ignored during
"split".
Combining pieces together
Each of the elementary pieces of regular expressions which were
described before (such as "ab" or "\Z") could match at most one sub-
string at the given position of the input string. However, in a typi-
cal regular expression these elementary pieces are combined into more
complicated patterns using combining operators "ST", "S|T", "S*" etc
(in these examples "S" and "T" are regular subexpressions).
Such combinations can include alternatives, leading to a problem of
choice: if we match a regular expression "a|ab" against "abc", will it
match substring "a" or "ab"? One way to describe which substring is
actually matched is the concept of backtracking (see "Backtracking").
However, this description is too low-level and makes you think in terms
of a particular implementation.
Another description starts with notions of "better"/"worse". All the
substrings which may be matched by the given regular expression can be
sorted from the "best" match to the "worst" match, and it is the "best"
match which is chosen. This substitutes the question of "what is cho-
sen?" by the question of "which matches are better, and which are
worse?".
Again, for elementary pieces there is no such question, since at most
one match at a given position is possible. This section describes the
notion of better/worse for combining operators. In the description
below "S" and "T" are regular subexpressions.
"ST"
Consider two possible matches, "AB" and "A'B'", "A" and "A'" are
substrings which can be matched by "S", "B" and "B'" are substrings
which can be matched by "T".
If "A" is better match for "S" than "A'", "AB" is a better match
than "A'B'".
If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B"
is better match for "T" than "B'".
"S|T"
When "S" can match, it is a better match than when only "T" can
match.
Ordering of two matches for "S" is the same as for "S". Similar
for two matches for "T".
"S{REPEAT_COUNT}"
Matches as "SSS...S" (repeated as many times as necessary).
"S{min,max}"
Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".
"S{min,max}?"
Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".
"S?", "S*", "S+"
Same as "S{0,1}", "S{0,BIG_NUMBER}", "S{1,BIG_NUMBER}" respec-
tively.
"S??", "S*?", "S+?"
Same as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?" respec-
tively.
"(?>S)"
Matches the best match for "S" and only that.
"(?=S)", "(?<=S)"
Only the best match for "S" is considered. (This is important only
if "S" has capturing parentheses, and backreferences are used some-
where else in the whole regular expression.)
"(?!S)", "(?at a given position.
One more rule is needed to understand how a match is determined for the
whole regular expression: a match at an earlier position is always bet-
ter than a match at a later position.
Creating custom RE engines
Overloaded constants (see overload) provide a simple way to extend the
functionality of the RE engine.
Suppose that we want to enable a new RE escape-sequence "\Y|" which
matches at boundary between whitespace characters and non-whitespace
characters. Note that "(?=\S)(? \&convert;
}
sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
# We must also take care of not escaping the legitimate \\Y|
# sequence, hence the presence of '\\' in the conversion rules.
my %rules = ( '\\' => '\\\\',
'Y|' => qr/(?=\S)(?;
chomp $re;
$re = customre::convert $re;
/\Y|$re\Y|/;
BUGS
This document varies from difficult to understand to completely and
utterly opaque. The wandering prose riddled with jargon is hard to
fathom in several places.
This document needs a rewrite that separates the tutorial content from
the reference content.
SEE ALSO
perlrequick.
perlretut.
"Regexp Quote-Like Operators" in perlop.
"Gory details of parsing quoted constructs" in perlop.
perlfaq6.
"pos" in perlfunc.
perllocale.
perlebcdic.
Mastering Regular Expressions by Jeffrey Friedl, published by O'Reilly
and Associates.
perl v5.8.8 2006-01-07 PERLRE(1)
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