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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 introduction
is available in perlrequick, and a longer tutorial introduction is
available in perlretut.
For reference on how regular expressions are used in matching operations,
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 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 $* 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 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 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 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 routines.) 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" 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 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 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 alphabetic character, 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 matching an English
word). If "use locale" is in effect, the list of alphabetic 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 perluniintro 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 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 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 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 \<digit> within the 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
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 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 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 produce
$1, $2, etc, so you also pay a price for each pattern that contains
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 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 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 parentheses 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 documentation on an
individual feature 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 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 subexpressions
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 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 follows a tab, without
including the tab in $&. Works only for fixed-width look-behind.
"(?<!pattern)"
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 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 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 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 assertion is backtracked; compare
"Backtracking".
For reasons of security, this construct is forbidden 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
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
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 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 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 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 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 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" 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 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 better
reflects the above specification 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.
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 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 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 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 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 <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> <$2>\n";
} else {
print "FAIL\n";
}
}
That will print out:
(.*)(\d*) <I have 2 numbers: 53147> <>
(.*)(\d+) <I have 2 numbers: 5314> <7>
(.*?)(\d*) <> <>
(.*?)(\d+) <I have > <2>
(.*)(\d+)$ <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 assertions 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 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 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, 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 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 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 quantifier
("\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 $y =~ /^(\D*)(?=\d)(?!123)/ ;
6: got ABC
In other words, the two zero-width assertions next to each other work as
though they're ANDed together, just as you'd use any built-in assertions:
"/^$/" 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 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 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 routines,
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 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 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 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 alternative
contains everything from the last "|" to the next pattern 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 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 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 subpattern 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:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the RHS of a substitute to avoid shocking the sed
addicts, but it's a dirty habit to get into. That's because in PerlThink,
the righthand side of an "s///" is a double-quoted string. "\1" in the
usual double-quoted string means a control-A. The customary Unix meaning
of "\1" is kludged in for "s///". However, if you get into the habit of
doing that, you get yourself into trouble if you then add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying "\{1}000", whereas you can fix it
with "${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
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 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 prohibition 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.
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 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 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 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}" 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 regular 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" actually 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.
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)(?<!\S)|(?!\S)(?<=\S)" matches exactly at these positions,
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 $re;
}
Now "use customre" enables the new escape in constant regular expressions,
i.e., those without any runtime variable interpolations. As documented in
overload, this conversion will work only over literal parts of regular
expressions. For "\Y|$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|/;
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.
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