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)^(?!.*?_.*?)(?!(?:[\w]+?\.)?\-[\w\.\-]*?)(?![\w]+?\-\.(?:[\w\.\-]+?))(?=[\w])(?=[\w\.\-]*?\.+[\w\.\-]*?)(?![\w\.\-]{254})(?!(?:\.?[\w\-\.]*?[\w\-]{64,}\.)+?)[\w\.\-]+?(?<![\w\-\.]*?\.[\d]+?)(?<=[\w\-]{2,})(?<![\w\-]{25})$`) var str = `# TEST ## List of valid domains: www.google.com google.com mkyong123.com mkyong-info.com sub.mkyong.com sub.mkyong-info.com mkyong.com.au g.co a.12E mkyong.t.t.co xn--stackoverflow.com stackoverflow.xn--com xn--d1ai6ai.xn--p1ai stackoverflow.co.uk a.xn--wgbh1c 1.2.3.4.com x.XN--VERMGENSBERATUNG-PWB abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com www.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcde.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com ## List of invalid domains: http://google.com http://www.google.com dot. space com under_score.com underscore.c_om -dash.com dash-.com sub.-dash.com sub-.dash.com -.com -com .com com mx.gmail.com. mkyong.t.t.c mkyong,com mkyong.com/users a.123 x.XN--VERMGENSBERATUNG-PWBB abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijkk.com www.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijkk.co.uk abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcde.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.comm 0123456789 +-.,!@#$%^&*();\\/|<>\"\' 12345 -98.7 3.141 .6180 9,000 +42 HELP https://github.com/c-hive/guides/blob/57cbbf72f1152214a160f9be8a57e2c4dc868cfd/etc/regex.md#domains # allowed chars [\w\d\.\-] EXPLAIN # once upon a time... ^ # doesn't contain underscore (this is so that we can use \w alter on) (?![\w\d\.\-]*?_[\w\d\.\-]*?) # doesn't contain dash at the beginning of a label (?!(?:[\d\w]+?\.)?\-[\w\d\.\-]*?) # doesn't contain dash at the end of a label (?![\w\d]+?\-\.(?:[\d\w\.\-]+?)) # starts with a non-limit char (?=[\w\d\]) # contains at least 1 dot (?=[\w\d\.\-]*?\.+[\w\d\.\-]*?) # not longer than 253 chars (?![\w\d\.\-]{253}) # doesn't contain a label longer than 63 char (?!(?:\.?[\w\d\-\.]*?[\w\d\-]{64,}\.)+?) # allowed chars [\w\d\.\-]+? # TLD is not digit-only (?<![\w\d\-\.]*?\.[\d]+?) # TLD is at least 2 characters (?<=[\w\d\-]{2,}) # TLD is at most 24 characters (?<![\w\d\-]{25}) # the end &` 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/