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perlre ()
>> perlre (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
perlre (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
NAME
perlre - Perl regular expressions
DESCRIPTION
This page describes the syntax of regular expressions in
Perl. For a description of how to use regular expressions
in matching operations, plus various examples of the same,
see discussions of `m//', `s///', `qr//' and `??' in the
Regexp Quote-Like Operators entry in the perlop manpage.
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 the
Regexp Quote-Like Operators entry in the perlop manpage and
the Gory details of parsing quoted constructs entry in the
perlop manpage.
i Do case-insensitive pattern matching.
If `use locale' is in effect, the case map is taken from
the current locale. See the perllocale manpage.
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 yet 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 the perlop manpage.
RegularExpressions
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
the Version 8 Regular Expressions entry elsewhere in this
document 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.) 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
the perllocale manpage. For documentation of `\N{name}',
see the charnames manpage.
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 C<(?:\PM\pM*)>
\C Match a single C char (octet) even under utf8.
A `\w' matches a single alphanumeric character, 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 the perllocale manpage. 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. See the utf8
manpage for details about `\pP', `\PP', and `\X'.
The POSIX character class syntax
[:class:]
is also available. The available classes and their
backslash equivalents (if available) are as follows:
alpha
alnum
ascii
cntrl
digit \d
graph
lower
print
punct
space \s
upper
word \w
xdigit
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 one, zero, any alphabetic character, and the
percentage sign.
If the `utf8' pragma is used, the following equivalences to
Unicode \p{} constructs hold:
alpha IsAlpha
alnum IsAlnum
ascii IsASCII
cntrl IsCntrl
digit IsDigit
graph IsGraph
lower IsLower
print IsPrint
punct IsPunct
space IsSpace
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 isalpha(3) interface (except
for `word', which is a Perl extension, mirroring `\w').
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.
graph
Any alphanumeric or punctuation character.
print
Any alphanumeric or punctuation character or space.
punct
Any punctuation character.
xdigit
Any hexadecimal digit. Though this may feel silly
(/0-9a-f/i 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 trad. Perl utf8 Perl
[:^digit:] \D \P{IsDigit}
[:^space:] \S \P{IsSpace}
[:^word:] \W \P{IsWord}
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 the Regexp Quote-Like
Operators entry in the perlop manpage. 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 the pos entry in the
perlfunc manpage.
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 9'th ASCII character, a tab.) 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. And `$'' returns everything after the matched
string.
The numbered variables ($1, $2, $3, etc.) and the related
punctuation set (`<$+', `$&', `$`', and `$'') are all
dynamically scoped until the end of the enclosing block or
until the next successful match, whichever comes first.
(See the Compound Statements entry in the perlsyn manpage.)
WARNING: Once Perl sees that you need one of `$&', `$`', or
`$'' 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-alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
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 the Gory details of parsing quoted
constructs entry in the perlop manpage.
ExtendedPatterns
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.
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/ ) { }
Letters after a `-' turn those modifiers off.
These modifiers are localized inside an enclosing
group (if any). For example,
( (?i) blah ) \s+ \1
will match a repeated (includingthecase!) 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 evaluate 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.
The `code' is properly scoped in the following
sense: If the assertion is backtracked (compare
the section on "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 the
section on "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
the re manpage), or the variables contain results
of `qr//' operator (see the qr/STRING/imosx entry
in the perlop manpage).
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
module. See the perlsec manpage 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 nothingotherthanthissubstring. This
construct is useful for optimizations of what
would otherwise be "eternal" matches, because it
will not backtrack (see the section on
"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 the
section on "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 the null string many times"'):
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
appearence, `#[ \t]*' isnot 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 the Combining pieces
together entry elsewhere in this document.
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 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 tricker. 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 it 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}){0,5}[c]/
And if you used `*''s instead of limiting it to 0 through 5
matches, then it would take forever--or until you ran out of
stack space.
A powerful tool for optimizing such beasts is what is known
as an "independent group", which does not backtrack (see the
section on "`(?>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 the section on "`(?>pattern)'".
Version8RegularExpressions
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.) 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 ASCII value is nnn. Similarly,
\xnn, where nn are hexadecimal digits, matches the character
whose ASCII value is nn. The expression \cx matches the
ASCII 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.
Warningon\1vs$1
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the RHS of a substitute to avoid
shocking the sed addicts, but it's a dirty habit to get
into. That's because in PerlThink, the righthand side of a
`s///' is a double-quoted string. `\1' in the usual
double-quoted string means a control-A. The customary Unix
meaning of `\1' is kludged in for `s///'. However, if you
get into the habit of doing that, you get yourself into
trouble if you then add an `/e' modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying `\{1}000', whereas you
can fix it with `${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///'.
Repeatedpatternsmatchingzero-lengthsubstringWARNING: 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 forcefullybreakingtheinfiniteloop. 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 the section on
"Backtracking"), and so the secondbest match is chosen if
the best match is of zero length.
For example:
$_ = 'bar';
s/\w??/<$&>/g;
results in `"<'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 matchedwithzero-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'.
Combiningpiecestogether
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 the section on
"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 atagivenposition. 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.
CreatingcustomREengines
Overloaded constants (see the overload manpage) 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 white-space
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]'"}
my %rules = ( '\\' => '\\',
'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
sub convert {
my $re = shift;
$re =~ s{
\\ ( \\ | Y . )
}
{ $rules{$1} or invalid($re,$1) }sgex;
return $re;
}
Now `use customre' enables the new escape in constant
regular expressions, i.e., those without any runtime
variable interpolations. As documented in the overload
manpage, this conversion will work only over literal parts
of regular expressions. For `\Y|$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
the Regexp Quote-Like Operators entry in the perlop manpage.
the Gory details of parsing quoted constructs entry in the
perlop manpage.
the perlfaq6 manpage.
the pos entry in the perlfunc manpage.
the perllocale manpage.
MasteringRegularExpressions by Jeffrey Friedl, published
by O'Reilly and Associates.
