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

Code Generator

Language

Generated Code

$re = '/^#+(.+)\n*((?:\n(?:^[^\n#].{0,2}$|^[^\n#].{3}(?<!<!--).*))+)/m'; $str = '# Style This style is suitable for having the header as the front, and the answer as the back 1 newline <!--ID: 1717098832201--> # Style This style is suitable for having the header as the front, and the answer as the back 1 newline 1 newline with blanks 2 newlines 2 newlines with blanks 2 newlines mixed blanks <!--ID: 1717098832201--> # Overall heading ## Subheading 1 You\'re allowed to nest headers within each other 1 newline 1 newline with blanks 2 newlines 2 newlines with blanks 2 newlines mixed blanks ## Subheading 2 It\'ll take the deepest level for the question 1 newline 1 newline with blanks 2 newlines 2 newlines with blanks 2 newlines mixed blanks ## Subheading 3 It\'ll even Span over Multiple lines, and ignore preceding whitespace 1 newline 1 newline with blanks 2 newlines 2 newlines with blanks 2 newlines mixed blanks '; preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0); // Print the entire match result var_dump($matches);

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