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preg_match("/^
    (1[-\s.])?	# optional '1-', '1.' or '1'
    ( \( )?		# optional opening parenthesis
    \d{3}		# the area code
    (?(2) \) )	# if there was opening parenthesis, close it
    [-\s.]?		# followed by '-' or '.' or space
    \d{3}		# first 3 digits
    [-\s.]?		# followed by '-' or '.' or space
    \d{4}		# last 4 digits
  $/x",$number);

Callbacks

$template = preg_replace_callback('/\[(.*)\]/','my_callback',$template);  
function my_callback($matches) {  
  /* codigo */
}

‘Codicia’ de la regexp

$html = '<a href="/hello">Hello</a> <a href="/world">World!</a>';  
// note the ?'s after the *'s  
if (preg_match_all('/<a.*?>.*?<\/a>/',$html,$matches)) {  
  print_r($matches);  
}

Prefijos y sufijos

$pattern = '/foo(?=bar)/'; 
$pattern = '/foo(?!bar)/'; 
$pattern = '/(?<!foo)bar/';

Condicionales

// if it begins with 'q', it must begin with 'qu'  
// else it must begin with 'f'  
$pattern = '/^(?(?=q)qu|f)/';

Subpatrones

preg_match('/(?P<hi>H.*) (?P<fstar>f.*)(?P<bstar>b.*)/', 'Hello foobar', $matches);  
echo "f* => " . $matches['fstar']; // prints 'f* => foo'  
echo "b* => " . $matches['bstar']; // prints 'b* => bar'  
 

Advanced Regular Expression Tips and Techniques

• Validar números reales

$values= array(10.75, '10.75', -1234.5678, 'This is a string', array());
$filteredValues = filter_var($values, FILTER_VALIDATE_FLOAT, FILTER_REQUIRE_ARRAY);

• Validar expresiones regulares

if(filter_var($value, FILTER_VALIDATE_REGEXP, array('options' => array('regexp' => "/^G(.+)/")))) {
// Lo que sea
}

• Validar URLs

$url = 'http://www.devshed.com';
if(filter_var($url, FILTER_VALIDATE_URL)) {
// Lo que sea
}

Working with Regular Expressions with Filters in PHP 5

• Usuario: está es sencilla, letras, dígitos y el subrayado, con una longitud entre 3 y 16 caracteres.

/^[a-zA-Z0-9_]{3,16}$/

• Etiquetas XHTML: habrá que sustituir la palabra etiqueta por la correspondiente que queremos buscar.

/]*>(.*?)<\/etiqueta>/

• Etiqueta con atributos: lo mismo que la anterior pero con atributos.

/]*atributo\s*=\s*(["'])valor\\1[^>]*>(.*?)<\/etiqueta>/

• Direcciones de correo: según la especificación RFC 822:

function is_valid_email_address($email){
$qtext = '[^\x0d\x22\x5c\x80-\xff]';
$dtext = '[^\x0d\x5b-\x5d\x80-\xff]';
$atom = '[^\x00-\x20\x22\x28\x29\x2c\x2e\x3a-\x3c'.
'\x3e\x40\x5b-\x5d\x7f-\xff]+';
$quoted_pair = '\x5c[\x00-\x7f]';
$domain_literal = "\x5b($dtext|$quoted_pair)*\x5d";
$quoted_string = "\x22($qtext|$quoted_pair)*\x22";
$domain_ref = $atom;
$sub_domain = "($domain_ref|$domain_literal)";
$word = "($atom|$quoted_string)";
$domain = "$sub_domain(\x2e$sub_domain)*";
$local_part = "$word(\x2e$word)*";
$addr_spec = "$local_part\x40$domain";
return preg_match("!^$addr_spec$!", $email) ? 1 : 0;
}

• URL: para reconocer direcciones de internet:

{
\b
# Parte principal (protocolo://dominio, o solo dominio)
(
# http://, o https://
(https?)://[-\w]+(\.\w[-\w]*)+
|
# Nombre de dominio
(?i: [a-z0-9] (?:[-a-z0-9]*[a-z0-9])? \. )+ # subdominions
# .com, .net...
(?-i: com\b
| edu\b
| biz\b
| gov\b
| in(?:t|fo)\b # .int or .info
| mil\b
| net\b
| org\b
| [a-z][a-z]\.[a-z][a-z]\b # two-letter country code
)
)
# Puerto
( : \d+ )?
# El resto es opcional que empieza por /
(
/
[^.!,?;"\'<>()[]{}sx7F-\xFF]*
(
[.!,?]+ [^.!,?;”\’<>()\[\]{\}s\x7F-\xFF]+
)*
)?
}ix

5 Regular Expressions Every Web Programmer Should Know

Vía / dzone