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

$re = '/^(?=.{1,255}$)(?=[^.]{1,63}(?![^.]))[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*(?:[.](?=[^.]{1,63}(?![^.]))[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*)+(?:[.][a-zA-Z0-9-]*[a-zA-Z0-9])?$/m'; $str = 'I\'m trying to build a regex where it accepts domain names with the following conditions: Allows DNS names (only hyphens, periods and alphanumeric characters allowed) upto 255 characters. Hyphens can only appear in between letters Should start/ end with a letter. The length of the label before a period should be 63 Possible Cases: aa a.b.c a-a.b aaaa.bbbb.ccc.1234567890123456789012345678901234567890123456789012345678901234567 63++++ Cases that should not pass a-.b qwertqwertqwertqwertqwertqwertqwertqwertqwertqwertqwertqwertqwer.v What I have built looks like this: But this does not accept a.b.c'; preg_match_all($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