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    Pattern Modifiers

    The current possible PCRE modifiers are listed below. The names in parentheses refer to internal PCRE names for these modifiers. Spaces and newlines are ignored in modifiers, other characters cause error.

    i (PCRE_CASELESS)
    If this modifier is set, letters in the pattern match both upper and lower case letters.
    m (PCRE_MULTILINE)
    By default, PCRE treats the subject string as consisting of a single "line" of characters (even if it actually contains several newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless D modifier is set). This is the same as Perl. When this modifier is set, the "start of line" and "end of line" constructs match immediately following or immediately before any newline in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m modifier. If there are no "\n" characters in a subject string, or no occurrences of ^ or $ in a pattern, setting this modifier has no effect.
    s (PCRE_DOTALL)
    If this modifier is set, a dot metacharacter in the pattern matches all characters, including newlines. Without it, newlines are excluded. This modifier is equivalent to Perl's /s modifier. A negative class such as [^a] always matches a newline character, independent of the setting of this modifier.
    x (PCRE_EXTENDED)
    If this modifier is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class, and characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x modifier, and makes it possible to include commentary inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.
    A (PCRE_ANCHORED)
    If this modifier is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the start of the string which is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl.
    D (PCRE_DOLLAR_ENDONLY)
    If this modifier is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this modifier, a dollar also matches immediately before the final character if it is a newline (but not before any other newlines). This modifier is ignored if m modifier is set. There is no equivalent to this modifier in Perl.
    S
    When a pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. If this modifier is set, then this extra analysis is performed. At present, studying a pattern is useful only for non-anchored patterns that do not have a single fixed starting character. As of PHP 7.3.0 this flag has no effect.
    U (PCRE_UNGREEDY)
    This modifier inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by ?. It is not compatible with Perl. It can also be set by a (?U) modifier setting within the pattern or by a question mark behind a quantifier (e.g. .*?).

    Note:

    It is usually not possible to match more than pcre.backtrack_limit characters in ungreedy mode.

    X (PCRE_EXTRA)
    This modifier turns on additional functionality of PCRE that is incompatible with Perl. Any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. There are at present no other features controlled by this modifier.
    J (PCRE_INFO_JCHANGED)
    The (?J) internal option setting changes the local PCRE_DUPNAMES option. Allow duplicate names for subpatterns. As of PHP 7.2.0 J is supported as modifier as well.
    u (PCRE_UTF8)
    This modifier turns on additional functionality of PCRE that is incompatible with Perl. Pattern and subject strings are treated as UTF-8. An invalid subject will cause the preg_* function to match nothing; an invalid pattern will trigger an error of level E_WARNING. Five and six octet UTF-8 sequences are regarded as invalid.
    n (PCRE_NO_AUTO_CAPTURE)
    This modifier makes simple (xyz) groups non-capturing. Only named groups like (?<name>xyz) are capturing. This only affects which groups are capturing, it is still possible to use numbered subpattern references, and the matches array will still contain numbered results. Available as of PHP 8.2.0.

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    User Contributed Notes 11 notes

    up
    27
    hfuecks at nospam dot org
    19 years ago
    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 http://www.tldp.org/ 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: http://hsivonen.iki.fi/php-utf8/

    The following script should give you an idea of what works and what doesn't;

    <?php
    $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";
    preg_match("/".$str."/u",'Testing');
    }

    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";
    }

    }
    ?>
    up
    13
    varrah NO_GARBAGE_OR_SPAM AT mail DOT ru
    19 years ago
    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.
    up
    11
    phpman at crustynet dot org dot uk
    13 years ago
    The description of the "u" flag is a bit misleading. It suggests that it is only required if the pattern contains UTF-8 characters, when in fact it is required if either the pattern or the subject contain UTF-8. Without it, I was having problems with preg_match_all returning invalid multibyte characters when given a UTF-8 subject string.

    It's fairly clear if you read the documentation for libpcre:

    In order process UTF-8 strings, you must build PCRE to include UTF-8
    support in the code, and, in addition, you must call pcre_compile()
    with the PCRE_UTF8 option flag, or the pattern must start with the
    sequence (*UTF8). When either of these is the case, both the pattern
    and any subject strings that are matched against it are treated as
    UTF-8 strings instead of strings of 1-byte characters.

    [from http://www.pcre.org/pcre.txt]
    up
    7
    arash dot dalir at gmail dot com
    7 years ago
    the PCRE_INFO_JCHANGED modifier is apparently not accepted as a global option (after the closing delimiter) in PHP versions <= 5.4 (not checked in PHP 5.5) but allowed in PHP 5.6 (also not checked in PHP 7.X)

    The following pattern doesn't work in PHP 5.4, but it works in PHP 5.6:

    <?php
    //test.php
    preg_match_all('/(?<dup_name>\d{1,4})\-(?<dup_name>\d{1,2})/J', '1234-23', $matches);
    var_dump($matches);

    /*
    output in PHP 5.4:
    Warning: preg_match_all(): Unknown modifier 'J' in test.php on line 3
    NULL
    --------------
    output PHP 5.6:
    array(4) {
    [0]=> array(1) { [0]=> string(7) "1234-23" }
    ["dup_name"]=> array(1) { [0]=> string(2) "23" }
    [1]=> array(1) { [0]=> string(4) "1234" }
    [2]=> array(1) { [0]=> string(2) "23" }
    }
    */
    ?>

    in order to resolve this issue in PHP 5.4, one can use the (?J) pattern modifier, which indicates the pattern (from that point forward) allows duplicate names for subpatterns.

    code which works in PHP 5.4:
    <?php

    preg_match_all    ('/(?J)(?<dup_name>\d{1,4})\-(?<dup_name>\d{1,2})/', '1234-23', $matches);
    var_dump($matches);

    /*
    output in PHP 5.4:
    array(4) {
    [0]=> array(1) { [0]=> string(7) "1234-23" }
    ["dup_name"]=> array(1) { [0]=> string(2) "23" }
    [1]=> array(1) { [0]=> string(4) "1234" }
    [2]=> array(1) { [0]=> string(2) "23" }
    }
    --------------
    output in PHP 5.6 (the same as with /J):
    array(4) {
    [0]=> array(1) { [0]=> string(7) "1234-23" }
    ["dup_name"]=> array(1) { [0]=> string(2) "23" }
    [1]=> array(1) { [0]=> string(4) "1234" }
    [2]=> array(1) { [0]=> string(2) "23" }
    }
    */
    ?>
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    4
    Hayley Watson
    4 years ago
    Starting from 7.3.0, the 'S' modifier has no effect; this analysis is now always done by the PCRE engine.
    up
    10
    Daniel Klein
    12 years ago
    If the _subject_ contains utf-8 sequences the 'u' modifier should be set, otherwise a pattern such as /./ could match a utf-8 sequence as two to four individual ASCII characters. It is not a requirement, however, as you may have a need to break apart utf-8 sequences into single bytes. Most of the time, though, if you're working with utf-8 strings you should use the 'u' modifier.

    If the subject doesn't contain any utf-8 sequences (i.e. characters in the range 0x00-0x7F only) but the pattern does, as far as I can work out, setting the 'u' modifier would have no effect on the result.
    up
    2
    Anonymous
    5 years ago
    A warning about the /i modifier and POSIX character classes:
    If you're using POSIX character classes in your regex that indicate case such as [:upper:] or [:lower:] in combination with the /i modifier, then in PHP < 7.3 the /i modifier will take precedence and effectively make both those character classes work as [:alpha:], but in PHP >= 7.3 the character classes overrule the /i modifier.
    up
    2
    Wirek
    6 years ago
    A hint for those of you who are trying to fight off (or work around at least) the problem of matching a pattern correctly at the end ($) of any line in multiple lines mode (/m).
    <?php
    // Various OS-es have various end line (a.k.a line break) chars:
    // - Windows uses CR+LF (\r\n);
    // - Linux LF (\n);
    // - OSX CR (\r).
    // And that's why single dollar meta assertion ($) sometimes fails with multiline modifier (/m) mode - possible bug in PHP 5.3.8 or just a "feature"(?).
    $str="ABC ABC\n\n123 123\r\ndef def\rnop nop\r\n890 890\nQRS QRS\r\r~-_ ~-_";
    // C 3 p 0 _
    $pat1='/\w$/mi'; // This works excellent in JavaScript (Firefox 7.0.1+)
    $pat2='/\w\r?$/mi';
    $pat3='/\w\R?$/mi'; // Somehow disappointing according to php.net and pcre.org
    $pat4='/\w\v?$/mi';
    $pat5='/(*ANYCRLF)\w$/mi'; // Excellent but undocumented on php.net at the moment
    $n=preg_match_all($pat1, $str, $m1);
    $o=preg_match_all($pat2, $str, $m2);
    $p=preg_match_all($pat3, $str, $m3);
    $r=preg_match_all($pat4, $str, $m4);
    $s=preg_match_all($pat5, $str, $m5);
    echo     $str."\n1 !!! $pat1 ($n): ".print_r($m1[0], true)
    .    "\n2 !!! $pat2 ($o): ".print_r($m2[0], true)
    .    "\n3 !!! $pat3 ($p): ".print_r($m3[0], true)
    .    "\n4 !!! $pat4 ($r): ".print_r($m4[0], true)
    .    "\n5 !!! $pat5 ($s): ".print_r($m5[0], true);
    // Note the difference among the three very helpful escape sequences in $pat2 (\r), $pat3 (\R), $pat4 (\v) and altered newline option in $pat5 ((*ANYCRLF)) - for some applications at least.

    /* The code above results in the following output:
    ABC ABC

    123 123
    def def
    nop nop
    890 890
    QRS QRS

    ~-_ ~-_
    1 !!! /\w$/mi (3): Array
    (
    [0] => C
    [1] => 0
    [2] => _
    )

    2 !!! /\w\r?$/mi (5): Array
    (
    [0] => C
    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
    )

    3 !!! /\w\R?$/mi (5): Array
    (
    [0] => C

    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
    )

    4 !!! /\w\v?$/mi (5): Array
    (
    [0] => C

    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
    )

    5 !!! /(*ANYCRLF)\w$/mi (7): Array
    (
    [0] => C
    [1] => 3
    [2] => f
    [3] => p
    [4] => 0
    [5] => S
    [6] => _
    )
    */
    ?>
    Unfortunately, I haven't got any access to a server with the latest PHP version - my local PHP is 5.3.8 and my public host's PHP is version 5.2.17.
    up
    1
    Wirek
    6 years ago
    An important addendum (with new $pat3_2 utilising \R properly, its results and comments):
    Note that there are (sometimes difficult to grasp at first glance) nuances of meaning and application of escape sequences like \r, \R and \v - none of them is perfect in all situations, but they are quite useful nevertheless. Some official PCRE control options and their changes come in handy too - unfortunately neither (*ANYCRLF), (*ANY) nor (*CRLF) is documented here on php.net at the moment (although they seem to be available for over 10 years and 5 months now), but they are described on Wikipedia ("Newline/linebreak options" at https://en.wikipedia.org/wiki/Perl_Compatible_Regular_Expressions) and official PCRE library site ("Newline convention" at http://www.pcre.org/original/doc/html/pcresyntax.html#SEC17) pretty well. The functionality of \R appears somehow disappointing (with default configuration of compile time option) according to php.net as well as official description ("Newline sequences" at https://www.pcre.org/original/doc/html/pcrepattern.html#newlineseq) when used improperly.

    A hint for those of you who are trying to fight off (or work around at least) the problem of matching a pattern correctly at the end (or at the beginning) of any line even without the multiple lines mode (/m) or meta-character assertions ($ or ^).
    <?php
    // Various OS-es have various end line (a.k.a line break) chars:
    // - Windows uses CR+LF (\r\n);
    // - Linux LF (\n);
    // - OSX CR (\r).
    // And that's why single dollar meta assertion ($) sometimes fails with multiline modifier (/m) mode - possible bug in PHP 5.3.8 or just a "feature"(?) of default configuration option for meta-character assertions (^ and $) at compile time of PCRE.
    $str="ABC ABC\n\n123 123\r\ndef def\rnop nop\r\n890 890\nQRS QRS\r\r~-_ ~-_";
    // C 3 p 0 _
    $pat3='/\w\R?$/mi'; // Somehow disappointing according to php.net and pcre.org when used improperly
    $pat3_2='/\w(?=\R)/i'; // Much better with allowed lookahead assertion (just to detect without capture) without multiline (/m) mode; note that with alternative for end of string ((?=\R|$)) it would grab all 7 elements as expected, but '/(*ANYCRLF)\w$/mi' is more straightforward in use anyway
    $p=preg_match_all($pat3, $str, $m3);
    $r=preg_match_all($pat3_2, $str, $m4);
    echo     $str."\n3 !!! $pat3 ($p): ".print_r($m3[0], true)
    .    "\n3_2 !!! $pat3_2 ($r): ".print_r($m4[0], true);
    // Note the difference between the two very helpful escape sequences in $pat3 and $pat3_2 (\R) - for some applications at least.

    /* The code above results in the following output:
    ABC ABC

    123 123
    def def
    nop nop
    890 890
    QRS QRS

    ~-_ ~-_
    3 !!! /\w\R?$/mi (5): Array
    (
    [0] => C

    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
    )

    3_2 !!! /\w(?=\R)/i (6): Array
    (
    [0] => C
    [1] => 3
    [2] => f
    [3] => p
    [4] => 0
    [5] => S
    )
    */
    ?>
    Unfortunately, I haven't got any access to a server with the latest PHP version - my local PHP is 5.3.8 and my public host's PHP is version 5.2.17.
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    2
    michal dot kocarek at brainbox dot cz
    15 years ago
    In case you're wondering, what is the meaning of "S" modifier, this paragraph might be useful:

    When "S" modifier is set, PHP calls the pcre_study() function from the PCRE API before executing the regexp. Result from the function is passed directly to pcre_exec().

    For more information about pcre_study() and "Studying the pattern" check the PCRE manual on http://www.pcre.org/pcre.txt

    PS: Note that function names "pcre_study" and "pcre_exec" used here refer to PCRE library functions written in C language and not to any PHP functions.
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    2
    ebarnard at marathonmultimedia dot com
    17 years ago
    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:

    <?php
    $target     = 'some text';
    if(    preg_match('/
    e # Comments here
    /x'    ,$target)) {
    print     "Target 1 hit.\n";
    }
    if(    preg_match('/
    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
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