Metacaracteres

El poder de las expresiones regulares viene dado por la capacidad de incluir alternativas y repeticiones en el patrón. Éstos están codificadoes en el patrón por el uso de metacaracteres, los cuales no se representan a sí mismos, sino que son interpretados de una forma especial.

Hay dos conjuntos diferentes de metacaracteres: aquéllos que son reconocidos en cualquier lugar de un patrón excepto dentro de los corchetes, y aquéllos que son reconocidos dentro de los corchetes. Fuera de los corchetes, los metacaracteres son los siguientes:

Metacaracteres fuera de corchetes
MetacarácterDescripción
\carácter de escape general con varios usos
^declaración de inicio de sujeto (o línea, en modo multilínea)
$declaración de fin de sujeto o antes de la terminación de nueva línea (o fin línea, en modo multilínea)
.coincide con cualquier carácter excepto con el de nueva línea (por defecto)
[inicio de la definición de la clase carácter
]fin de la definición de la clase carácter
|inicio de rama alternativa
(inicio de sub-patrón
)fin de sub-patrón
?amplia el significado de (, también cuantificador 0 o 1, también hace perezosos a los cuantificadores codiciosos (véase repetición)
*cuantificador 0 o más
+cuantificador 1 o más
{inicio de cuantificador mín/máx
}fin de cuantificador mín/máx
La parte de un patrón que está entre corchetes se llama una clase carácter. En una clase carácter los únicos metacaracteres son:
Metacaracteres dentro de corchetes (clases carácter)
MetacarácterDescripción
\carácter de escape general
^niega la clase, pero sólo si se trata del primer carácter
-indica el rango de caracteres
Las siguientes secciones describen el uso de cada metacarácter.

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User Contributed Notes 5 notes

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2
Wirek
7 months 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.
up
0
Wirek
7 months ago
Significantly updated version (with $pat4 and the best $pat5!):
A hint for those of you who are trying to fight off (or work around at least) the problem of matching a pattern correctly at the end ($) 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';
$pat2='/\w\r?$/mi';
$pat3='/\w\R?$/mi';
$pat4='/\w\v?$/mi';
$pat5='/(*ANYCRLF)\w$/mi';
$n=preg_match_all($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 between 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$/mi (7): Array
(
    [0] => C
    [1] => 3
    [2] => f
    [3] => p
    [4] => 0
    [5] => S
    [6] => _
)
*/
?>
Unfortunately, I haven't got any access to a server with the latest PHP version - my local PHP is 5.3.8 and my public host's PHP is version 5.2.17.
up
-1
Wirek
7 months ago
A hint for those of you who are trying to fight off (or work around at least) the problem of matching a pattern correctly at the end ($) 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(?).
$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';
$pat2='/\w\r?$/mi';
$pat3='/\w\R?$/mi';
$n=preg_match_all($pat1, $str, $m1);
$o=preg_match_all($pat2, $str, $m2);
$p=preg_match_all($pat3, $str, $m3);
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);
// Note the difference between the two very helpful escape sequences in $pat2 (\r) and in $pat3 (\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

~-_ ~-_
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] => _
)
*/
?>
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
-2
Kurt Wei
2 years ago
disturbing usage of "any character" for multi-lines...

remark:
'.' (all characters) just does NOT include on single character the newline (\n) by default,
while \n is included in all other matching searches (e.g. \s).
Funny enough, the "carriage return" (\r) is included, when using '.'

You have to write "(.|\\n)" instead of a single dot, with disadvantages in using complex matching-results,

or simple use the "s" modificator to bring dot to accept the newline.

$subject="<tag>Hello\nWorld</tag>";

preg_match( '/<tag>[A-Za-z\\s]*<\\/tag>/' , $subject ); //true
preg_match( '/<tag>[^<]*<\\/tag>/' , $subject ); //true
preg_match( '/<tag>(.|\\n)*<\\/tag>/' , $subject ); //true
preg_match( '/<tag>.*<\\/tag>/s' , $subject ); //true
preg_match( '/<tag>.*<\\/tag>/' , $subject ); //ATTENTION! *false*
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-33
Thomas
3 years ago
The meta character $ accepts a (one) newline character (\n).

(Take a moment to let this information sink in)

You might want to (r)trim() your input afterwards if you have a match because otherwise it (still) might not meet a length requirement or other strange stuff might happen when you store the input as-is.
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