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Man Page for PERLRE
NAME
perlre - Perl regular expressions
DESCRIPTION
For a description of how to use regular expressions in
matching operations, see m// and s/// in the perlop manpage.
The matching operations can have various modifiers, some of
which relate to the interpretation of the regular expression
inside. These are:
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
s Treat string as single line.
x Use extended regular expressions.
These are usually written as "the /x modifier", even though
the delimiter in question might not actually be a slash. In
fact, any of these modifiers may also be embedded within the
regular expression itself using the new (?...) construct.
See below.
The /x modifier itself needs a little more explanation. It
tells the regular expression parser to ignore whitespace
that is not backslashed or within a character class. You
can use this to break up your regular expression into
(slightly) more readable parts. Together with the
capability of embedding comments described later, this goes
a long way towards making Perl 5 a readable language. See
the C comment deletion code in the perlop manpage.
Regular Expressions
The patterns used in pattern matching are regular
expressions such as those supplied in the Version 8 regexp
routines. (In fact, the routines are derived (distantly)
from Henry Spencer's freely redistributable reimplementation
of the V8 routines.) See the section on 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
| Alternation
() Grouping
[] Character class
By default, the "^" character is guaranteed to match only at
the beginning of the string, the "$" character only at the
end (or before the newline at the end) and Perl does certain
optimizations with the assumption that the string contains
only one line. Embedded newlines will not be matched by "^"
or "$". 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
deprecated in Perl 5.)
To facilitate multi-line substitutions, the "." character
never matches a newline unless you use the /s modifier,
which 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}. There is no limit to the size of n or m,
but large numbers will chew up more memory.
By default, a quantified subpattern is "greedy", that is, it
will match as many times as possible without causing the
rest pattern not to match. The standard quantifiers are all
"greedy", in that they match as many occurrences as possible
(given a particular starting location) without causing the
pattern to fail. If you want it to match the minimum number
of times possible, follow the quantifier with a "?" after
any of them. Note that the meanings don't change, just the
"gravity":
*? 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
Since patterns are processed as double quoted strings, the
following also work:
\t tab
\n newline
\r return
\f form feed
\v vertical tab, whatever that is
\a alarm (bell)
\e escape
\033 octal char
\x1 hex char
\l lowercase next char
\c[ uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
\Q quote regexp metacharacters till \E
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
Note that \w matches a single alphanumeric character, not a
whole word. To match a word you'd need to say \w+. You may
use \w, \W, \s, \S, \d and \D within character classes
(though not as eieher end of a range).
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
\G Match only where previous m//g left off
A word boundary (\b) is defined as a spot between two
characters that has a \w on one side of it and 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.) The \A and \Z are
just like "^" and "$" except that they won't match multiple
times when the /m modifier is used, while "^" and "$" will
match at every internal line boundary.
When the bracketing construct ( ... ) is used, \
matches the digit'th substring. (Outside of the pattern,
always use "$" instead of "\" in front of the digit. The
scope of $ (and $`, $&, and $') extends to the end of
the enclosing BLOCK or eval string, or to the next pattern
match with subexpressions. If you want to use parentheses to
delimit subpattern (e.g. a set of alternatives) without
saving it as a subpattern, follow the ( with a ?. The
<digit> notation sometimes works outside the current
pattern, but should not be relied upon.) You may have as
many parentheses as you wish. If you have more than 9
substrings, the variables $10, $11, ... refer to the
corresponding substring. Within the pattern, 10, 11, etc.
refer back to substrings if there have been at least that
many left parens before the backreference. Otherwise (for
backward compatibilty) 10 is the same as 10, a backspace,
and 11 the same as 11, a tab. And so on. (1 through 9
are always backreferences.)
$+ returns whatever the last bracket match matched. $&
returns the entire matched string. ($0 used to return the
same thing, but not any more.) $` returns everything before
the matched string. $' returns everything after the matched
string. Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
You will note that all 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 makes it
simple to quote a string that you want to use for a pattern
but that you are afraid might contain metacharacters.
Simply quote all the non-alphanumeric characters:
$pattern =~ s/(W)/\$1/g;
You can also use the built-in quotemeta() function to do
this. An even easier way to quote metacharacters right in
the match operator is to say
/$unquotedQ$quotednquoted/
Perl 5 defines a consistent extension syntax for regular
expressions. The syntax is a pair of parens with a question
mark as the first thing within the parens (this was a syntax
error in Perl 4). The character after the question mark
gives the function of the extension. Several extensions are
already supported:
(?#text) A comment. The text is ignored.
(?:regexp)
This groups things like "()" but doesn't make
backrefences like "()" does. So
split(/\b(?:a|b|c)\b/)
is like
split(/\b(a|b|c)\b/)
but doesn't spit out extra fields.
(?=regexp)
A zero-width positive lookahead assertion. For
example, /\w+(?=\t)/ matches a word followed by a
tab, without including the tab in $&.
(?!regexp)
A zero-width negative lookahead assertion. For
example /foo(?!bar)/ matches any occurrence of
"foo" that isn't followed by "bar". Note however
that lookahead and lookbehind are NOT the same
thing. You cannot use this for lookbehind:
/(?!foo)bar/ will not find an occurrence of "bar"
that is preceded by something which is not "foo".
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)...|^..?)bar/.
Sometimes it's still easier just to say:
if (/foo/ && $` =~ /bar$/)
(?imsx) One or more embedded pattern-match modifiers.
This is particularly useful for patterns that are
specified in a table somewhere, some of which want
to be case sensitive, and some of which don't.
The case insensitive ones merely need to include
(?i) at the front of the pattern. For example:
$pattern = "foobar";
if ( /$pattern/i )
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ )
The specific choice of question mark for this and the new
minimal matching construct was because 1) question mark is
pretty rare in older regular expressions, and 2) whenever
you see one, you should stop and "question" exactly what is
going on. That's psychology...
Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8
regexp 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 which normally function as
metacharacters to be interpreted literally by prefixing them
with a "
matches a "
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 of the characters
in the list. If the first character after the "[" is "^",
the class matches any character not in the list. Within a
list, the "-" character is used to specify a range, so that
a-z represents all the characters between "a" and "z",
inclusive.
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 hexidecimal 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). Note that 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. For this reason, it's common
practice to include alternatives in parentheses, to minimize
confusion about where they start and end. Note also that
the pattern (fee|fie|foe) differs from the pattern
[fee|fie|foe] in that the former matches "fee", "fie", or
"foe" in the target string, while the latter matches
anything matched by the classes [fee], [fie], or [foe] (i.e.
the class [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. Note that
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", since subpattern 1
actually matched "0x", even though the rule [0|0x] could
potentially match the leading 0 in the second number.
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