Regular Expressions 101

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An explanation of your regex will be automatically generated as you type.
Detailed match information will be displayed here automatically.
  • All Tokens
  • Common Tokens
  • General Tokens
  • Anchors
  • Meta Sequences
  • Quantifiers
  • Group Constructs
  • Character Classes
  • Flags/Modifiers
  • Substitution
  • A single character of: a, b or c
    [abc]
  • A character except: a, b or c
    [^abc]
  • A character in the range: a-z
    [a-z]
  • A character not in the range: a-z
    [^a-z]
  • A character in the range: a-z or A-Z
    [a-zA-Z]
  • Any single character
    .
  • Alternate - match either a or b
    a|b
  • Any whitespace character
    \s
  • Any non-whitespace character
    \S
  • Any digit
    \d
  • Any non-digit
    \D
  • Any word character
    \w
  • Any non-word character
    \W
  • Non-capturing group
    (?:...)
  • Capturing group
    (...)
  • Zero or one of a
    a?
  • Zero or more of a
    a*
  • One or more of a
    a+
  • Exactly 3 of a
    a{3}
  • 3 or more of a
    a{3,}
  • Between 3 and 6 of a
    a{3,6}
  • Start of string
    ^
  • End of string
    $
  • A word boundary
    \b
  • Non-word boundary
    \B

Regular Expression
No Match

/
/
gm

Test String

Substitution

Processing...

Code Generator

Language

Generated Code

$re = '/(?<=^|[^_])__(((\\\\_)|((?![\s_])(?:(?:[^_]|_(?!_))+?)?[^\s_]))+)__(?=[^_]|$)/m'; $str = '-- funciona __foo bar__ __f o o b a r__ **teste __lorem ipsum__ dolor sit** __teste **lorem ipsum** dolor sit__ abc__xyz__teste -- não funcionava na primeira versão __foo_bar__ __a__ __*__ __foo_*bar__ -- não é para funcionar __ foo bar__ __foo bar __ _foo__ __ foo __ _foo_ __foo_ __ __ __a____ ____a__ _____ -- corner case: aninhado (não tratei pois não sei o que deveria fazer) __abc__xyz__123__ __abc __xyz__ 123__ -- corner case: escape (chega a reconhecer alguns casos, mas não substitui "\\_" por "_" __\\___ __a\\___ __abc\\___ __\\_abc_\\__ -- errado, mas não consegui tratar'; $subst = "<i>\1<\i>"; $result = preg_replace($re, $subst, $str); echo "The result of the substitution is ".$result;

Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php