?? in Regexp Quote-Like Operators.
The matching operations can have various modifiers. The modifiers that relate to the interpretation of the regular expression inside are listed below. For the modifiers that alter the way a regular expression is used by Perl, see Regexp Quote-Like Operators and Gory details of parsing quoted constructs.
If use locale is in effect, the case
map is taken from the current locale. See the perllocale
manpage.
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.
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
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
of 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.
The patterns used in pattern matching are regular expressions such as those supplied in the Version 8 regex routines. (In fact, 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 at only
the
beginning of the string, the ``$'' character at only the end (or before the
newline at
the end) and Perl does certain optimizations with the assumption
that the string contains
only one line. Embedded newlines will not be
matched by ``^'' or ``$''. 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 facilitate 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
65536.
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
\c[ control 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.
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
A \w matches a single
alphanumeric character, not a whole word. To match a word
you'd need to say
\w+. 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 (though not as either 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, or before newline at the end
\z Match only at end of string
\G Match only
where previous m//g left off (works only with /g)
A word
boundary (\b) is defined as 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.) 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, not ignoring newline,
you
can use \z. The \G assertion can be used to chain global
matches (using m//g), as described in
Regexp Quote-Like Operators.
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.
When the bracketing construct ( ... ) is
used, \<digit> matches the digit'th substring. Outside of the pattern, always use
``$'' instead of ``\'' in front of the digit. (While the \<digit> notation can on rare
occasion work outside the current pattern, this should not be relied upon. See the
WARNING below.) The scope of $<digit> (and
$`,
$&, and $') extends to the end of the enclosing
BLOCK or eval string, or to the next successful
pattern match, whichever
comes first. If you want to use parentheses to
delimit a subpattern (e.g., a set of
alternatives) without saving it as a
subpattern, follow the ( with a ?:.
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 parentheses before the backreference. Otherwise (for backward compatibility) \10 is the same as \010, a backspace, and \11 the same as \011, 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;
}
Once perl sees that you need one of $&, $` or
$' anywhere in the program, it has to provide them on each and every
pattern
match. This can slow your program down. The same mechanism that handles
these
provides for the use of $1, $2, etc., so you pay the same price for
each pattern that
contains capturing parentheses. But if you never use
$&, etc., in your script, then
patterns without capturing parentheses won't 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;
Now it is much more common to see either the
quotemeta() function or
the \Q escape sequence used to disable
all metacharacters' special meanings like
this:
/$unquoted\Q$quoted\E$unquoted/
Perl defines a consistent extension syntax for regular expressions. The syntax is a pair of parentheses with a question mark as the first thing within the parentheses (this was a syntax error in older versions of Perl). The character after the question mark gives the function of the extension. Several extensions are already supported:
/x switch is used to enable 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.
@fields = split(/\b(?:a|b|c)\b/)
is like
@fields = split(/\b(a|b|c)\b/)
but doesn't spit out extra fields.
The letters between
? and : act as flags modifiers,
see
(?imsx-imsx). In particular,
/(?s-i:more.*than).*million/i
is equivalent to more verbose
/(?:(?s-i)more.*than).*million/i
/\w+(?=\t)/
matches a word followed
by a tab, without including the tab in $&.
/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.
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 lookbehind see below.
/(?<=\t)\w+/
matches
a word following a tab, without including the tab in $&. Works only for
fixed-width lookbehind.
/(?<!bar)foo/
matches any
occurrence of ``foo'' that isn't following ``bar''. Works only
for fixed-width
lookbehind.
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 Backtracking), all the changes
introduced after
localisation are undone, so
$_ = '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 0, since the scopes which restrict local statements are unwound.
This assertion may be used
as (?(condition)yes-pattern)
switch. If not used in this way, the result
of evaluation of code
is put into variable
$^R. This
happens immediately, so
$^R can be used from other
(?{ code
}) assertions inside the same regular expression.
The above assignment to $^R is properly localized, thus the old value of $^R is restored if the assertion is backtracked (compare Backtracking).
Due to security concerns, this construction is
not allowed if the regular
expression involves run-time interpolation of variables, unless
use re 'eval' pragma is used (see the re
manpage), or the variables contain results of
qr() operator (see qr/STRING/imosx).
This restriction is due to the wide-spread (questionable) practice of using the construct
$re = <>;
chomp $re;
$string =~ /$re/;
without tainting. While this code is frowned
upon from security point of
view, when (?{}) was introduced, it was considered
bad to add
new security holes to existing scripts.
NOTE:
Use of the above insecure snippet without also enabling taint mode is to be
severely frowned
upon. use re 'eval' does not disable tainting checks, thus to allow
$re in the
above snippet to contain (?{})
with tainting
enabled, one needs both use re 'eval' and untaint the
$re.
pattern would match if anchored at the given position,
and only this
substring.
Say, ^(?>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
(?=(pattern))\1
since
the lookahead is in "logical" context, thus matches the same substring as a
standalone a+. The following \1 eats the matched string, thus
making a zero-length assertion into an
analogue of
(?>...). (The difference
between these two constructs is that the second one uses
a catching group, thus shifting
ordinals of backreferences in the rest of a
regular expression.)
This construct is useful for optimizations of ``eternal'' matches, because it will not backtrack (see Backtracking).
m{ \(
(
[^()]+
|
\( [^()]* \)
)+
\)
}x
That will efficiently match a nonempty group with
matching
two-or-less-level-deep parentheses. 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 that the
above
pattern 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 modification of this pattern
m{ \(
(
(?> [^()]+ )
|
\(
[^()]* \)
)+
\)
}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
as. Be aware, however, that this pattern currently triggers a warning
message
under
-w saying it "matches the null string many
times"):
On simple groups, such as the pattern (? [^()]+
)>, a comparable effect may be achieved by negative lookahead, as
in [^()]+ (?!
[^()] ). This was only 4 times slower on a string with 1000000
as.
(condition) should be either an integer in parentheses (which is
valid if the
corresponding pair of parentheses matched), or
lookahead/lookbehind/evaluate
zero-width assertion.
Say,
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly included in parentheses themselves.
(?i) at the front of the pattern. For
example:
$pattern = "foobar";
if ( /$pattern/i ) { }
# more flexible:
$pattern =
"(?i)foobar";
if ( /$pattern/ ) { }
Letters after -
switch modifiers off.
These modifiers are localized inside an enclosing group (if any). Say,
( (?i) blah ) \s+ \1
(assuming x modifier, and no
i modifier outside of this group) will match a repeated (including the
case!) word blah in any case.
A question mark was chosen for this and for the new minimal-matching construct 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...
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}?.
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''. In this case, 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 lookahead 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 of
course 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 in fact
``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
by a digit, and in
fact, it must also be followed by something that's not ``123''.
Remember
that the lookaheads 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 $y =~ /^(\D*)(?=\d)(?!123)/
;
6: got ABC
In other words, the two zero-width assertions
next to each other work as
though they're ANDed together, just as you'd use any builtin
assertions: /^$/
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.
One warning: particularly complicated regular expressions can take exponential time to solve due to the immense number of possible ways they can use backtracking to try match. For example this will take a very long time to run
/((a{0,5}){0,5}){0,5}/
And if you used *'s instead of limiting it to
0 through 5 matches, then it would take
literally forever--or until you ran out of stack
space.
A powerful tool for optimizing such beasts
is
``independent'' groups, which do not backtrace (see
(?>pattern)). Note also that zero-length lookahead/lookbehind
assertions will not
backtrace to make the tail match, since they are in ``logical''
context:
only the fact whether they match or not is considered relevant. For an
example
where side-effects of a lookahead might have influenced the following match, see
(?>pattern).
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 is used to specify a range, so that a-z represents
all characters between ``a'' and ``z'', inclusive. If you want
``-'' itself to be a member
of a class, put it at the start or end of the
list, or escape it with a backslash. (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.)
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. For this reason, 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
mathing 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 actually matched ``0x'', even though the rule
0|0x could potentially match the leading 0 in the second
number.
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
${1}000. Basically, the operation of interpolation should not be confused
with the
operation of matching a backreference. Certainly they mean two different
things on
the left side of the s///.
WARNING: Difficult material (and prose) ahead. This section needs a rewrite.
Regular expressions provide a terse and powerful programming language. As with most other power tools, power comes together with the ability to wreak havoc.
A common abuse of this power stems from the ability to make infinite loops using regular expressions, with something as innocous as:
'foo' =~ m{ ( o? )* }x;
The o? can match
at the beginning of 'foo', and since the position in the string is not moved by
the match, o? would match again and again due to the * modifier.
Another common way to create a similar cycle is with the looping
modifier
//g:
@matches = ( 'foo' =~ m{ o? }xg );
or
print "match: <$&>\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 which may match zero-length substrings, with a simple example being:
@chars = split //,
$string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; #
parens avoid magic s// /
Thus Perl allows the /()/ construct, which
forcefully breaks
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 when it is detected that a repeated expression did match 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.
Say,
$_ = 'bar';
s/\w??/<$&>/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 to the matched string, and is reset by each assignment to
pos().
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|/;
Gory details of parsing quoted constructs.
pos.
Mastering Regular Expressions (see the perlbook manpage) by Jeffrey Friedl.