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

package main import ( "regexp" "fmt" ) func main() { var re = regexp.MustCompile(`(?m)<<<([A-Za-z_]\w*)(?:\R(?!\1;\R).*)*\R(?:\R(?!\1;\R).*)*\R\1;\R`) var str = `<<<([A-Z]+)\n.*\n\n.*\n *\1\b This works if there is only one HEREDOC of that type in the file, but if there were more than one it'll look at the code between the HEREDOCs. <<<([A-Z]+)\n(?!.*\1.*).*\n\n(?!.*\1.*).*\n *\1\b With negative lookaheads should then work... but it doesn't match at all :-( I think I can't use negative look-behinds cause it would not contain a fixed length. $out .= <<<HTML aaaaa aaaaa bbbbb HTML; // Shouldn't match, cause the empty line is outside the HEREDOC $out .= <<<HTML aaaaa aaaaa bbbbb HTML; $out .= <<<HTML Should match, cause it has a empty line in the HEREDOC aaaaa bbbbb HTML; ` for i, match := range re.FindAllString(str, -1) { fmt.Println(match, "found at index", i) } }

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 Golang, please visit: https://golang.org/pkg/regexp/