Perl-Compatible Regular Expressions
Perl has long been considered the benchmark for powerful
regular expressions. PHP uses a C library called pcre
to provide almost complete support for
Perl’s arsenal of regular expression features. Perl regular expressions
include the POSIX classes and anchors described earlier. A POSIX-style
character class in a Perl regular expression works and understands
non-English characters using the Unix locale system. Perl regular
expressions act on arbitrary binary data, so you can safely match with
patterns or strings that contain the NUL-byte (\x00
).
Delimiters
Perl-style regular expressions emulate the Perl syntax for
patterns, which means that each pattern must be enclosed in a pair of
delimiters . Traditionally, the slash (/
) character is used; for example, /
pattern
/
. However, any non-alphanumeric
character other than the backslash character (\
) can be used to delimit a Perl-style
pattern. This is useful when matching strings containing slashes, such
as filenames. For example, the following are equivalent:
preg_match('/\/usr\/local\//', '/usr/local/bin/perl'); // returns true preg_match('#/usr/local/#', '/usr/local/bin/perl'); // returns true
Parentheses (( )
), curly
braces ({}
), square brackets
([]
), and angle brackets (<>
) can be used as pattern
delimiters:
preg_match('{/usr/local/}', '/usr/local/bin/perl'); // returns true
The section "Trailing
Options" discusses the single-character modifiers you can put
after the closing delimiter to modify the behavior of the regular
expression engine. A very useful one is x
, which makes the regular expression engine
strip whitespace and #
-marked
comments from the regular expression before matching. These two
patterns are the same, but one is much easier to read:
'/([[:alpha:]]+)\s+\1/' '/( # start capture [[:alpha:]]+ # a word \s+ # whitespace \1 # the same word again ) # end capture /x'
Match Behavior
While Perl’s regular expression syntax includes the POSIX constructs we talked about earlier, some pattern components have a different meaning in Perl. In particular, Perl’s regular expressions are optimized for matching against single lines of text (although there are options that change this behavior).
The period (.
) matches any
character except for a newline (\n
). The dollar sign ($
) matches at the end of the string or, if
the string ends with a newline, just before that newline:
preg_match('/is (.*)$/', "the key is in my pants", $captured); // $captured[1] is 'in my pants'
Character Classes
Perl-style regular expressions support the POSIX character classes but also define some of their own, as shown in Table 4-9.
Anchors
Perl-style regular expressions also support additional anchors , as listed in Table 4-10.
Assertion | Meaning |
| Word boundary (between |
| Non-word boundary (between |
| Beginning of string |
| End of string or before |
| End of string |
| Start of line (or after |
| End of line (or before |
Quantifiers and Greed
The POSIX quantifiers , which Perl also supports, are always greedy . That is, when faced with a quantifier, the engine matches as much as it can while still satisfying the rest of the pattern. For instance:
preg_match('/(<.*>)/', 'do <b>not</b> press the button', $match); // $match[1] is '<b>not</b>'
The regular expression matches from the first less-than sign to
the last greater-than sign. In effect, the .*
matches everything after the first
less-than sign, and the engine backtracks to make it match less and
less until finally there’s a greater-than sign to be matched.
This greediness can be a problem. Sometimes you need
minimal (non-greedy) matching—that is,
quantifiers that match as few times as possible to satisfy the rest of
the pattern. Perl provides a parallel set of quantifiers that match
minimally. They’re easy to remember, because they’re the same as the
greedy quantifiers , but with a question mark (?
) appended. Table 4-11 shows the
corresponding greedy and non-greedy quantifiers supported by
Perl-style regular expressions.
Here’s how to match a tag using a non-greedy quantifier:
preg_match('/(<.*?>)/', 'do <b>not</b> press the button', $match); // $match[1] is '<b>'
Another, faster way is to use a character class to match every non-greater-than character up to the next greater-than sign:
preg_match('/(<[^>]*>)/', 'do <b>not</b> press the button', $match); // $match[1] is '<b>'
Non-Capturing Groups
If you enclose a part of a pattern in parentheses, the text that
matches that subpattern is captured and can be accessed later.
Sometimes, though, you want to create a subpattern without capturing
the matching text. In Perl-compatible regular expressions, you can do
this using the (?:
subpattern
)
construct:
preg_match('/(?:ello)(.*)/', 'jello biafra', $match); // $match[1] is ' biafra'
Backreferences
You can refer to text captured earlier in a pattern with
a backreference: \1
refers to the contents of the first
subpattern, \2
refers to the
second, and so on. If you nest subpatterns, the first begins with the
first opening parenthesis, the second begins with the second opening
parenthesis, and so on.
For instance, this identifies doubled words:
preg_match('/([[:alpha:]]+)\s+\1/', 'Paris in the the spring', $m); // returns true and $m[1] is 'the'
You can’t capture more than 99 subpatterns.
Trailing Options
Perl-style regular expressions let you put single-letter options
(flags) after the regular expression pattern to modify the
interpretation, or behavior, of the match. For instance, to match
case-insensitively, simply use the i
flag:
preg_match('/cat/i', 'Stop, Catherine!'); // returns true
Table 4-12 shows the modifiers from Perl that are supported in Perl-compatible regular expressions.
Modifier | Meaning |
| Match case-insensitively. |
| Make period ( |
| Remove whitespace and comments from the pattern. |
| Make caret ( |
| If the replacement string is PHP
code, |
PHP’s Perl-compatible regular expression functions also support other modifiers that aren’t supported by Perl, as listed in Table 4-13.
Modifier | Meaning |
| Reverses the greediness of the
subpattern; |
| Causes pattern strings to be treated as UTF-8 |
| Causes a backslash followed by a character with no special meaning to emit an error |
| Causes the beginning of the string
to be anchored as if the first character of the pattern were
|
| Causes the |
| Causes the expression parser to more carefully examine the structure of the pattern, so it may run slightly faster the next time (such as in a loop) |
It’s possible to use more than one option in a single pattern, as demonstrated in the following example:
$message = <<< END To: you@youcorp From: me@mecorp Subject: pay up Pay me or else! END; preg_match('/^subject: (.*)/im', $message, $match); // $match[1] is 'pay up'
Inline Options
In addition to specifying pattern-wide options after the closing pattern delimiter, you can specify options within a pattern to have them apply only to part of the pattern. The syntax for this is:
(?flags
:subpattern
)
For example, only the word “PHP” is case-insensitive in this example:
preg_match('/I like (?i:PHP)/', 'I like pHp'); // returns true
The i
, m
, s
,
U
, x
, and X
options can be applied internally in this fashion. You can use
multiple options at once:
preg_match('/eat (?ix:fo o d)/', 'eat FoOD'); // returns true
Prefix an option with a hyphen (-
) to turn it off:
preg_match('/(?-i:I like) PHP/i', 'I like pHp'); // returns true
An alternative form enables or disables the flags until the end of the enclosing subpattern or pattern:
preg_match('/I like (?i)PHP/', 'I like pHp'); // returns true preg_match('/I (like (?i)PHP) a lot/', 'I like pHp a lot', $match); // $match[1] is 'like pHp'
Inline flags do not enable capturing. You need an additional set of capturing parentheses do that.
Lookahead and Lookbehind
In patterns it’s sometimes useful to be able to say “match here if this is next.” This is particularly common when you are splitting a string. The regular expression describes the separator, which is not returned. You can use lookahead to make sure (without matching it, thus preventing it from being returned) that there’s more data after the separator. Similarly, lookbehind checks the preceding text.
Lookahead and lookbehind come in two forms: positive and negative. A positive lookahead or lookbehind says “the next/preceding text must be like this.” A negative lookahead or lookbehind says “the next/preceding text must not be like this.” Table 4-14 shows the four constructs you can use in Perl-compatible patterns. None of the constructs captures text.
Construct | Meaning |
| Positive lookahead |
| Negative lookahead |
| Positive lookbehind |
| Negative lookbehind |
A simple use of positive lookahead is splitting a Unix mbox mail
file into individual messages. The word "From"
starting a line by itself indicates
the start of a new message, so you can split the mailbox into messages
by specifying the separator as the point where the next text is
"From"
at the start of a
line:
$messages = preg_split('/(?=^From )/m', $mailbox);
A simple use of negative lookbehind is to extract quoted strings
that contain quoted delimiters. For instance, here’s how to extract a
single-quoted string (note that the regular expression is commented
using the x
modifier):
$input = <<< END name = 'Tim O\'Reilly'; END; $pattern = <<< END ' # opening quote ( # begin capturing .*? # the string (?<! \\\\ ) # skip escaped quotes ) # end capturing ' # closing quote END; preg_match( "($pattern)x", $input, $match); echo $match[1]; Tim O\'Reilly
The only tricky part is that to get a pattern that looks behind to see if the last character was a backslash, we need to escape the backslash to prevent the regular expression engine from seeing “\),” which would mean a literal close parenthesis. In other words, we have to backslash that backslash: “\\).” But PHP’s string-quoting rules say that \\ produces a literal single backslash, so we end up requiring four backslashes to get one through the regular expression! This is why regular expressions have a reputation for being hard to read.
Perl limits lookbehind to constant-width expressions. That is, the expressions cannot contain quantifiers, and if you use alternation, all the choices must be the same length. The Perl-compatible regular expression engine also forbids quantifiers in lookbehind, but does permit alternatives of different lengths.
Cut
The rarely used once-only subpattern, or cut , prevents worst-case behavior by the regular expression engine on some kinds of patterns. The subpattern is never backed out of once matched.
The common use for the once-only subpattern is when you have a repeated expression that may itself be repeated:
/(a+|b+)*\.+/
This code snippet takes several seconds to report failure:
$p = '/(a+|b+)*\.+$/'; $s = 'abababababbabbbabbaaaaaabbbbabbababababababbba..!'; if (preg_match($p, $s)) { echo "Y"; } else { echo "N"; }
This is because the regular expression engine tries all the
different places to start the match, but has to backtrack out of each
one, which takes time. If you know that once something is matched it
should never be backed out of, you should mark it with (?>
subpattern
)
:
$p = '/(?>a+|b+)*\.+$/';
The cut never changes the outcome of the match; it simply makes it fail faster.
Conditional Expressions
A conditional expression is like an if
statement in a regular expression. The
general form is:
(?(condition
)yespattern
) (?(condition
)yespattern
|nopattern
)
If the assertion succeeds, the regular expression engine matches
the yespattern
. With the second form, if
the assertion doesn’t succeed, the regular expression engine skips the
yespattern
and tries to match the
nopattern
.
The assertion can be one of two types: either a backreference, or a lookahead or lookbehind match. To reference a previously matched substring, the assertion is a number from 1-99 (the most backreferences available). The condition uses the pattern in the assertion only if the backreference was matched. If the assertion is not a backreference, it must be a positive or negative lookahead or lookbehind assertion.
Functions
There are five classes of functions that work with Perl-compatible regular expressions : matching, replacing, splitting, filtering, and a utility function for quoting text.
Matching
The preg_match( )
function
performs Perl-style pattern matching on a string. It’s the
equivalent of the m//
operator in
Perl. The preg_match( )
function
takes the same arguments and gives the same return value as the
ereg( )
function, except that it
takes a Perl-style pattern instead of a standard pattern:
$found = preg_match(pattern
,string
[,captured
]);
For example:
preg_match('/y.*e$/', 'Sylvie'); // returns true preg_match('/y(.*)e$/', 'Sylvie', $m); // $m is array('ylvie', 'lvi')
While there’s an eregi( )
function to match case-insensitively, there’s no preg_matchi( )
function. Instead, use the
i
flag on the pattern:
preg_match('y.*e$/i', 'SyLvIe'); // returns true
The preg_match_all( )
function repeatedly matches from where the last match ended, until
no more matches can be made:
$found = preg_match_all(pattern
,string
,matches
[,order
]);
The order
value, either PREG_PATTERN_ORDER
or PREG_SET_ORDER
, determines the layout of
matches
. We’ll look at both, using this
code as a guide:
$string = <<< END 13 dogs 12 rabbits 8 cows 1 goat END; preg_match_all('/(\d+) (\S+)/', $string, $m1, PREG_PATTERN_ORDER); preg_match_all('/(\d+) (\S+)/', $string, $m2, PREG_SET_ORDER);
With PREG_PATTERN_ORDER
(the default), each element of the array corresponds to a particular
capturing subpattern. So $m1[0]
is an array of all the substrings that matched the pattern, $m1[1]
is an array of all the substrings
that matched the first subpattern (the numbers), and $m1[2]
is an array of all the substrings
that matched the second subpattern (the words). The array $m1
has one more elements than
subpatterns.
With PREG_SET_ORDER
, each
element of the array corresponds to the next attempt to match the
whole pattern. So $m2[0]
is an
array of the first set of matches ('13
dogs'
, '13'
, 'dogs'
), $m2[1]
is an array of the second set of
matches ('12 rabbits'
, '12'
, 'rabbits'
), and so on. The array $m2
has as many elements as there were
successful matches of the entire pattern.
Example 4-2 fetches the HTML at a particular web address into a string and extracts the URLs from that HTML. For each URL, it generates a link back to the program that will display the URLs at that address.
<?php if (getenv('REQUEST_METHOD') == 'POST') { $url = $_POST[url]; } else { $url = $_GET[url]; } ?> <form action="<?php echo $PHP_SELF ?>" method="POST"> URL: <input type="text" name="url" value="<?php echo $url ?>" /><br> <input type="submit"> </form> <?php if ($url) { $remote = fopen($url, 'r'); $html = fread($remote, 1048576); // read up to 1 MB of HTML fclose($remote); $urls = '(http|telnet|gopher|file|wais|ftp)'; $ltrs = '\w'; $gunk = '/#~:.?+=&%@!\-'; $punc = '.:?\-'; $any = "$ltrs$gunk$punc"; preg_match_all("{ \b # start at word boundary $urls : # need resource and a colon [$any] +? # followed by one or more of any valid # characters—but be conservative # and take only what you need (?= # the match ends at [$punc]* # punctuation [^$any] # followed by a non-URL character | # or $ # the end of the string ) }x", $html, $matches); printf("I found %d URLs<P>\n", sizeof($matches[0])); foreach ($matches[0] as $u) { $link = $PHP_SELF . '?url=' . urlencode($u); echo "<A HREF='$link'>$u</A><BR>\n"; } } ?>
Replacing
The preg_replace( )
function behaves like the search and replace operation in your text
editor. It finds all occurrences of a pattern in a string and
changes those occurrences to something else:
$new = preg_replace(pattern
,replacement
,subject
[,limit
]);
The most common usage has all the argument strings except for
the integer limit
. The limit is the
maximum number of occurrences of the pattern to replace (the
default, and the behavior when a limit of -1
is passed, is all occurrences).
$better = preg_replace('/<.*?>/', '!', 'do <b>not</b> press the button'); // $better is 'do !not! press the button'
Pass an array of strings as subject
to make the substitution on all of them. The new strings are
returned from preg_replace(
)
:
$names = array('Fred Flintstone', 'Barney Rubble', 'Wilma Flintstone', 'Betty Rubble'); $tidy = preg_replace('/(\w)\w* (\w+)/', '\1 \2', $names); // $tidy is array ('F Flintstone', 'B Rubble', 'W Flintstone', 'B Rubble')
To perform multiple substitutions on the same string or array
of strings with one call to preg_replace(
)
, pass arrays of patterns and replacements:
$contractions = array("/don't/i", "/won't/i", "/can't/i"); $expansions = array('do not', 'will not', 'can not'); $string = "Please don't yell—I can't jump while you won't speak"; $longer = preg_replace($contractions, $expansions, $string); // $longer is 'Please do not yell—I can not jump while you will not speak';
If you give fewer replacements than patterns, text matching the extra patterns is deleted. This is a handy way to delete a lot of things at once:
$html_gunk = array('/<.*?>/', '/&.*?;/'); $html = 'é : <b>very</b> cute'; $stripped = preg_replace($html_gunk, array( ), $html); // $stripped is ' : very cute'
If you give an array of patterns but a single string replacement, the same replacement is used for every pattern:
$stripped = preg_replace($html_gunk, '', $html);
The replacement can use backreferences. Unlike backreferences
in patterns, though, the preferred syntax for backreferences in
replacements is $1
, $2
, $3
,
etc. For example:
echo preg_replace('/(\w)\w+\s+(\w+)/', '$2, $1.', 'Fred Flintstone') Flintstone, F.
The /e
modifier makes
preg_replace( )
treat the
replacement string as PHP code that returns the actual string to use
in the replacement. For example, this converts every Celsius
temperature to Fahrenheit:
$string = 'It was 5C outside, 20C inside'; echo preg_replace('/(\d+)C\b/e', '$1*9/5+32', $string); It was 41 outside, 68 inside
This more complex example expands variables in a string:
$name = 'Fred'; $age = 35; $string = '$name is $age'; preg_replace('/\$(\w+)/e', '$$1', $string);
Each match isolates the name of a variable ($name
, $age
). The $1
in the replacement refers to those
names, so the PHP code actually executed is $name
and $age
. That code evaluates to the value of
the variable, which is what’s used as the replacement. Whew!
A variation on preg_replace(
)
is preg_replace_callback(
)
. This calls a function to get the replacement string.
The function is passed an array of matches (the zeroth element is
all the text that matched the pattern, the first is the contents of
the first captured subpattern, and so on). For example:
function titlecase ($s) { return ucfirst(strtolower($s[0])); } $string = 'goodbye cruel world'; $new = preg_replace_callback('/\w+/', 'titlecase', $string); echo $new; Goodbye Cruel World
Splitting
Whereas you use preg_match_all(
)
to extract chunks of a string when you know what those
chunks are, use preg_split( )
to
extract chunks when you know what separates the
chunks from each other:
$chunks = preg_split(pattern
,string
[,limit
[,flags
]]);
The pattern
matches a separator
between two chunks. By default, the separators are not returned. The
optional limit
specifies the maximum
number of chunks to return (-1
is
the default, which means all chunks). The
flags
argument is a bitwise OR
combination of the flags PREG_SPLIT_NO_EMPTY
(empty chunks are not
returned) and PREG_SPLIT_DELIM_CAPTURE
(parts of the
string captured in the pattern are returned).
For example, to extract just the operands from a simple numeric expression, use:
$ops = preg_split('{[+*/-]}', '3+5*9/2'); // $ops is array('3', '5', '9', '2')
To extract the operands and the operators, use:
$ops = preg_split('{([+*/-])}', '3+5*9/2', -1, PREG_SPLIT_DELIM_CAPTURE); // $ops is array('3', '+', '5', '*', '9', '/', '2')
An empty pattern matches at every boundary between characters in the string. This lets you split a string into an array of characters:
$array = preg_split('//', $string);
Filtering an array with a regular expression
The preg_grep( )
function returns those elements of an array that match a given
pattern:
$matching = preg_grep(pattern
,array
);
For instance, to get only the filenames that end in .txt, use:
$textfiles = preg_grep('/\.txt$/', $filenames);
Quoting for regular expressions
The preg_quote( )
function
creates a regular expression that matches only a given
string:
$re = preg_quote(string
[,delimiter
]);
Every character in string
that has
special meaning inside a regular expression (e.g., *
or $
)
is prefaced with a backslash:
echo preg_quote('$5.00 (five bucks)'); \$5\.00 \(five bucks\)
The optional second argument is an extra character to be quoted. Usually, you pass your regular expression delimiter here:
$to_find = '/usr/local/etc/rsync.conf'; $re = preg_quote($filename, '/'); if (preg_match("/$re", $filename)) { // found it! }
Differences from Perl Regular Expressions
Although very similar, PHP’s implementation of Perl-style regular expressions has a few minor differences from actual Perl regular expressions:
The null character (ASCII 0) is not allowed as a literal character within a pattern string. You can reference it in other ways, however (
\000
,\x00
, etc.).The
\E
,\G
,\L
,\l
,\Q
,\u
, and\U
options are not supported.The
(?{
some perl code
})
construct is not supported.The /
D
, /G
, /U
, /u
, /A
, and /X
modifiers are supported.The vertical tab
\v
counts as a whitespace character.Lookahead and lookbehind assertions cannot be repeated using
*
,+
, or?
.Parenthesized submatches within negative assertions are not remembered.
Alternation branches within a lookbehind assertion can be of different lengths.
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