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PHP : Function Reference : Regular Expression Functions (Perl-Compatible) : Pattern Modifiers

Pattern Modifiers

Describes possible modifiers in regex patterns ()

Code Examples / Notes » reference.pcre.pattern.modifiers


When adding comments with the /x modifier, don't use the pattern delimiter in the comments. It may not be ignored in the comments area. Example:
$target = 'some text';
               e # Comments here
              /x',$target)) {
   print "Target 1 hit.\n";
               e # /Comments here with slash
              /x',$target)) {
   print "Target 1 hit.\n";
prints "Target 1 hit." but then generates a PHP warning message for the second preg_match():
Warning:  preg_match() [function.preg-match]: Unknown modifier 'C' in /ebarnard/x-modifier.php on line 11

varrah no_garbage_or_spam

Spent a few days, trying to understand how to create a pattern for Unicode chars, using the hex codes. Finally made it, after reading several manuals, that weren't giving any practical PHP-valid examples. So here's one of them:
For example we would like to search for Japanese-standard circled numbers 1-9 (Unicode codes are 0x2460-0x2468) in order to make it through the hex-codes the following call should be used:
preg_match('/[\x{2460}-\x{2468}]/u', $str);
Here $str is a haystack string
\x{hex} - is an UTF-8 hex char-code
and /u is used for identifying the class as a class of Unicode chars.
Hope, it'll be useful.


Regarding the validity of a UTF-8 string when using the /u pattern modifier, some things to be aware of;
1. If the pattern itself contains an invalid UTF-8 character, you get an error (as mentioned in the docs above - "UTF-8 validity of the pattern is checked since PHP 4.3.5"
2. When the subject string contains invalid UTF-8 sequences / codepoints, it basically result in a "quiet death" for the preg_* functions, where nothing is matched but without indication that the string is invalid UTF-8
3. PCRE regards five and six octet UTF-8 character sequences as valid (both in patterns and the subject string) but these are not supported in Unicode ( see section 5.9 "Character Encoding" of the "Secure Programming for Linux and Unix HOWTO" - can be found at and other places )
4. For an example algorithm in PHP which tests the validity of a UTF-8 string (and discards five / six octet sequences) head to:
The following script should give you an idea of what works and what doesn't;
$examples = array(
   'Valid ASCII' => "a",
   'Valid 2 Octet Sequence' => "\xc3\xb1",
   'Invalid 2 Octet Sequence' => "\xc3\x28",
   'Invalid Sequence Identifier' => "\xa0\xa1",
   'Valid 3 Octet Sequence' => "\xe2\x82\xa1",
   'Invalid 3 Octet Sequence (in 2nd Octet)' => "\xe2\x28\xa1",
   'Invalid 3 Octet Sequence (in 3rd Octet)' => "\xe2\x82\x28",
   'Valid 4 Octet Sequence' => "\xf0\x90\x8c\xbc",
   'Invalid 4 Octet Sequence (in 2nd Octet)' => "\xf0\x28\x8c\xbc",
   'Invalid 4 Octet Sequence (in 3rd Octet)' => "\xf0\x90\x28\xbc",
   'Invalid 4 Octet Sequence (in 4th Octet)' => "\xf0\x28\x8c\x28",
   'Valid 5 Octet Sequence (but not Unicode!)' => "\xf8\xa1\xa1\xa1\xa1",
   'Valid 6 Octet Sequence (but not Unicode!)' => "\xfc\xa1\xa1\xa1\xa1\xa1",
echo "++Invalid UTF-8 in pattern\n";
foreach ( $examples as $name => $str ) {
   echo "$name\n";
echo "++ preg_match() examples\n";
foreach ( $examples as $name => $str ) {
   preg_match("/\xf8\xa1\xa1\xa1\xa1/u", $str, $ar);
   echo "$name: ";
   if ( count($ar) == 0 ) {
       echo "Matched nothing!\n";
   } else {
       echo "Matched {$ar[0]}\n";
echo "++ preg_match_all() examples\n";
foreach ( $examples as $name => $str ) {
   preg_match_all('/./u', $str, $ar);
   echo "$name: ";
   $num_utf8_chars = count($ar[0]);
   if ( $num_utf8_chars == 0 ) {
       echo "Matched nothing!\n";
   } else {
       echo "Matched $num_utf8_chars character\n";


Extracting lines of text:
You might want to grab a line of text within a multiline piece of text.  For example, suppose you want to replace the first and last lines within the <body> portion of a web $page with your own $lineFirst and $lineLast.  Here's one possible way:
$lineFirst = "This is a new first line
$lineLast  = "This is a new last line
$page = <<<EOD
<title>This is a test page</title>
This is the first line
Hi Fred
Hi Bill
This is the last line
$re = "/<body>.*^(.+)(^.*?^)(.+)(^<\\/body>.*?)/smU";
if (preg_match($re, $page, $aMatch, PREG_OFFSET_CAPTURE))
$newPage = substr($text, 0, $aMatch[1][1]) .
          $lineFirst . $aMatch[2][0] .
          $lineLast . $aMatch[4][0];
print $newPage;
The two (.+) are supposed to match the first and last lines within the <body> tag.  The /s option (dot all) is needed so the .* can also match newlines.  The /m option (multiline) is needed so that the ^ can match newlines.  The /U option (ungreedy) is needed so that the .* and .+ will only gobble up the minimum number of characters necessary to get to the character following the * or +.  The exception to this, however, is that the .*? temporarily overrides the /U setting on .* turning it from non greedy to greedy.  In the middle, this ensures that all the lines except the first and last (within the <body> tag) are put into $aMatch[2].  At the end, it ensures that all the remaining characters in the string are gobbled up, which could also have been achieved by .*)\\z/ instead of .*?)/
Csaba Gabor from Vienna

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