PERLRE(1) Perl Programmers Reference Guide PERLRE(1)NAMEperlre - 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 introduction is available in perlrequick, and a longer
tutorial introduction is available in perlretut.
For reference on how regular expressions are used in match-
ing operations, plus various examples of the same, see dis-
cussions 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 current 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 $* set-
ting. 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 whi-
tespace and comments.
These are usually written as "the "/x" modifier", even
though the delimiter in question might not really be a
slash. Any of these modifiers may also be embedded within
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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 backslashed 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 char-
acter 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 sup-
plied in the Version 8 regex routines. (The routines are
derived (distantly) from Henry Spencer's freely redistribut-
able reimplementation of the V8 routines.) See "Version 8
Regular Expressions" for details.
In particular the following metacharacters have their stan-
dard 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.)
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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" modifier 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 values less than a
preset limit defined when perl is built. This is usually
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 pat-
tern 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:
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\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 documentation 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 alpha-
betic character, or a decimal digit) or "_", not a whole
word. Use "\w+" to match a string of Perl-identifier char-
acters (which isn't the same as matching an English word).
If "use locale" is in effect, the list of alphabetic charac-
ters generated by "\w" is taken from the current locale.
See perllocale. You may use "\w", "\W", "\s", "\S", "\d",
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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 perluni-
intro 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 equivalents (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 char-
acters. 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 percen-
tage sign.
The following equivalences to Unicode \p{} constructs and
equivalent backslash character classes (if available), will
hold:
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[:...:] \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 output 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
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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 Opera-
tors" 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 sup-
ported 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 prob-
lems, 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 \<digit> within the
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match. Outside the match use "$" instead of "\". (The
\<digit> 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 horizontal 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 backrefer-
ences.
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 pro-
gram.) $` returns everything before the matched string. $'
returns everything after the matched string. And $^N con-
tains whatever was matched by the most-recently closed group
(submatch). $^N can be used in extended patterns (see
below), for example to assign a submatch to a variable.
The numbered match variables ($1, $2, $3, etc.) and the
related punctuation set ($+, $&, $`, $', 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 vari-
ables, which makes it easier to write code that tests for a
series of more specific cases and remembers the best match.
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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 produce $1, $2, etc, so you
also pay a price for each pattern that contains capturing
parentheses. (To avoid this cost while retaining the group-
ing behaviour, use the extended regular expression "(?: ...
)" instead.) But if you never use $&, $` or $', then pat-
terns 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 expres-
sion 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
interpolated variables) between "\Q" and "\E", double-
quotish backslash interpolation may lead to confusing
results. If you need to use literal backslashes within
"\Q...\E", consult "Gory details of parsing quoted con-
structs" 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 parentheses with a question mark as the first
thing within the parentheses. The character after the ques-
tion 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 documentation on an individual feature
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to verify its current status.
A question mark was chosen for this and for the minimal-
matching construct 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 com-
ment 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 some-
where, 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
subexpressions like "()", but doesn't make
backreferences as "()" does. So
@fields = split(/\b(?:a|b|c)\b/)
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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 cannot 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 fol-
lows a tab, without including the tab in $&. Works
only for fixed-width look-behind.
"(?<!pattern)"
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A zero-width negative look-behind assertion. For
example "/(?<!bar)foo/" matches any occurrence of
"foo" that does not follow "bar". Works only for
fixed-width look-behind.
"(?{ code })"
WARNING: This extended regular expression feature
is considered highly experimental, and may be
changed or deleted without notice.
This zero-width assertion evaluates any embedded
Perl code. It always succeeds, and its "code" is
not interpolated. Currently, the rules to deter-
mine where the "code" ends are somewhat convo-
luted.
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 regular 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
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are unwound.
This assertion may be used as a
"(?(condition)yes-pattern|no-pattern)" switch. If
not used in this way, the result of evaluation of
"code" is put into the special variable $^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 asser-
tion is backtracked; compare "Backtracking".
For reasons of security, this construct is forbid-
den if the regular expression involves run-time
interpolation of variables, 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 considered highly experimental, and may be
changed or deleted without notice. A simplified
version of the syntax may be introduced for com-
monly 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
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construct.
The "code" is not interpolated. As before, the
rules to determine where the "code" ends are
currently somewhat convoluted.
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 considered highly experimental, and may be
changed or deleted without notice.
An "independent" subexpression, one which matches
the substring 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 begin-
ning 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 particular, "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 fol-
lowing "\1" eats the matched string; it therefore
makes a zero-length assertion into an analogue of
"(?>...)". (The difference between these two
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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
currently 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 comparable effect may be achieved by nega-
tive look-ahead, as in "[^()]+ (?! [^()] )". This
was only 4 times slower on a string with 1000000
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"a"s.
The "grab all you can, and do not give anything
back" semantic 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 subex-
pression 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 better reflects the above specifica-
tion of comments.
"(?(condition)yes-pattern|no-pattern)"
"(?(condition)yes-pattern)"
WARNING: This extended regular expression feature
is considered highly experimental, and may be
changed or deleted without 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/evaluate zero-width
assertion.
For example:
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly
included in parentheses themselves.
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Backtracking
NOTE: This section presents an abstract approximation of
regular expression behavior. For a more rigorous (and com-
plicated) 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}?". Back-
tracking 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 containing a quantifier succeeds in a
way that causes later parts in the pattern to fail, the
matching engine backs up and recalculates the beginning
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 expres-
sion ("\b(foo)") finds a possible match right at the begin-
ning 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 character 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 out-
put of "table follows foo."
Sometimes minimal matching can help a lot. Imagine you'd
like to match everything between "foo" and "bar". Ini-
tially, you write something like this:
$_ = "The food is under the bar in the barn.";
if ( /foo(.*)bar/ ) {
print "got <$1>\n";
}
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Which perhaps unexpectedly yields:
got <d is under the bar in the >
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 <d is under the >
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 regular expression matched successfully.
Beginning is <I have 2 numbers: 53147>, 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> <$2>\n";
} else {
print "FAIL\n";
}
}
That will print out:
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(.*)(\d*) <I have 2 numbers: 53147> <>
(.*)(\d+) <I have 2 numbers: 5314> <7>
(.*?)(\d*) <> <>
(.*?)(\d+) <I have > <2>
(.*)(\d+)$ <I have 2 numbers: 5314> <7>
(.*?)(\d+)$ <I have 2 numbers: > <53147>
(.*)\b(\d+)$ <I have 2 numbers: > <53147>
(.*\D)(\d+)$ <I have 2 numbers: > <53147>
As you see, this can be a bit tricky. It's important to
realize that a regular expression is merely a set of asser-
tions that gives a definition of success. There may be 0,
1, or several different ways that the definition might
succeed against a particular string. And if there are mul-
tiple ways it might succeed, you need to understand back-
tracking 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 followed 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, con-
trary 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 general purpose version of test 1. The important
difference between them is that test 3 contains a quantifier
("\D*") and so can use backtracking, whereas test 1 will
not. What's happening is that you've asked "Is it true that
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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 pat-
tern 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 quantifier ("\D*") has been used in the regu-
lar 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 nega-
tion. 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 $y =~ /^(\D*)(?=\d)(?!123)/;
6: got ABC
In other words, the two zero-width assertions next to each
other work as though they're ANDed together, just as you'd
use any built-in assertions: "/^$/" matches only if you're
at the beginning of the line AND the end of the line simul-
taneously. The deeper underlying truth is that juxtaposi-
tion 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 exponential time to solve because of the immense number
of possible ways they can use backtracking to try match.
For example, without internal optimizations done by the reg-
ular 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 lim-
iting them to 0 through 5 matches, then it would take
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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 routines, here are the pattern-matching rules not
described above.
Any single character matches itself, unless it is a meta-
character with a special meaning described here or above.
You can cause characters that normally function as metachar-
acters 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 backslash. "-" 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 endpoints of a range,
that's not a range, the "-" is understood literally.
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 prin-
ciple is to use only ranges that begin from and end at
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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 expres-
sion \cx matches the character control-x. Finally, the "."
metacharacter matches any character 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 alternative includes everything
from the last pattern delimiter ("(", "[", or the beginning
of the pattern) up to the first "|", and the last alterna-
tive contains everything from the last "|" to the next pat-
tern delimiter. 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 chosen. This means that alternatives are
not necessarily greedy. For example: when matching
"foo|foot" against "barefoot", only the "foo" part will
match, as that is the first alternative tried, and it suc-
cessfully 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 brackets, 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 subpattern later in the pattern using
the metacharacter \n. Subpatterns are numbered based on the
left to right order of their opening parenthesis. 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 poten-
tially match the leading 0 in the second number.
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Warning on \1 vs $1
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the RHS of a substitute to avoid
shocking the sed addicts, but it's a dirty habit to get
into. That's because in PerlThink, the righthand side of an
"s///" is a double-quoted string. "\1" in the usual
double-quoted string means a control-A. The customary Unix
meaning of "\1" is kludged in for "s///". However, if you
get into the habit of doing that, you get yourself into
trouble if you then add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying "\{1}000", whereas you
can fix it with "${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
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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() opera-
tor.
The lower-level loops are interrupted (that is, the loop is
broken) when Perl detects that a repeated expression matched
a zero-length substring. 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 following match after a zero-length
match is prohibited to have a length of zero. This prohibi-
tion interacts with backtracking (see "Backtracking"), and
so the second best match is chosen if the best match is of
zero length.
For example:
$_ = 'bar';
s/\w??/<$&>/g;
results in "<><b><><a><><r><>". 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.
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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 substring at the given position of the input
string. However, in a typical regular expression these ele-
mentary 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 con-
cept 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 chosen?" 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 com-
bining 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.
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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 neces-
sary).
"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}"
respectively.
"S??", "S*?", "S+?"
Same as "S{0,1}?", "S{0,BIG_NUMBER}?",
"S{1,BIG_NUMBER}?" respectively.
"(?>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 somewhere else in the whole reg-
ular expression.)
"(?!S)", "(?<!S)"
For this grouping operator there is no need to describe
the ordering, since only whether or not "S" can match is
important.
"(??{ EXPR })"
The ordering is the same as for the regular expression
which is the result of EXPR.
"(?(condition)yes-pattern|no-pattern)"
Recall that which of "yes-pattern" or "no-pattern" actu-
ally matches is already determined. The ordering of the
matches is the same as for the chosen subexpression.
The above recipes describe the ordering of matches 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 better than a match
at a later position.
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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 charac-
ters and non-whitespace characters. Note that
"(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at these posi-
tions, so we want to have each "\Y|" in the place of the
more complicated version. We can create a module "customre"
to do this:
package customre;
use overload;
sub import {
shift;
die "No argument to customre::import allowed" if @_;
overload::constant 'qr' => \&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)(?<!\S)|(?!\S)(?<=\S)/ );
sub convert {
my $re = shift;
$re =~ s{
\\ ( \\ | Y . )
}
{ $rules{$1} or invalid($re,$1) }sgex;
return $re;
}
Now "use customre" enables the new escape in constant regu-
lar expressions, i.e., those without any runtime variable
interpolations. As documented in overload, this conversion
will work only over literal parts of regular expressions.
For "\Y|$re\Y|" the variable part of this regular expression
needs to be converted explicitly (but only if the special
meaning of "\Y|" should be enabled inside $re):
use customre;
$re = <>;
chomp $re;
$re = customre::convert $re;
/\Y|$re\Y|/;
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This document varies from difficult to understand to com-
pletely 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 2006-06-30 28
