Regular Expressions 101

Save & Share

Flavor

  • PCRE2 (PHP >=7.3)
  • PCRE (PHP <7.3)
  • ECMAScript (JavaScript)
  • Python
  • Golang
  • Java 8
  • .NET 7.0 (C#)
  • Rust
  • Regex Flavor Guide

Function

  • Match
  • Substitution
  • List
  • Unit Tests

Tools

Sponsors
There are currently no sponsors. Become a sponsor today!
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

/
/
gmx

Test String

Substitution

Processing...

Code Generator

Language

Generated Code

$re = '/([ :]) # look for : and space as possible leading characters ([\"]?) # if there\'s a quote, disregard it ([0-9]{4}) # find the year - # find the 1st dash ( # Group (4) ( ((0[13578] | (10|12)) # look for 31 day months - # followed by the 2nd dash (0[1-9]|[1-2][0-9]|3[0-1]) # and accept days up to 31 ) | # OR... (02 # look for Feb - # followed by the 2nd dash (0[1-9]|[1-2][0-9])) # and accept up to 29 days | # OR... ((0[469]|11)) # look for 30 day months - # followed by the 2nd dash ((0[1-9]|[1-2][0-9]|30)) # and accept up to 30 days ) ) # closes (4) ([\"]?)([ \"]?)(\s)/mx'; $str = 'This regex changes YYYY-MM-DD to "[[YYYY-MM-DD]]" for Obsidian WORKS ON: today: 2025-03-14 birthdays: 1966-06-19 Good Dates: 2022-12-31 and 2025-02-08 old-dates: 1492-01-01 old dates could be an issue, depends on how you use your vault! no space after colon is ok:2023-07-08 standing alone and quoted is fine: "2021-02-28" IGNORES: my template version number has dots for separators (skip): 2025.12.02 No leading whitespace is questionable?2023-06-08 existing links (no leading : or space): - [[2025-01-01]] - [2025-01-01](https:123.com) - "[[2023-11-30]]" - [["2024-06-05"]] Not YYYY-MM-DD dates:- 2022-1-1 Euro date: 2022-31-12 or slashes: 2021/09/15 Bad date for month: 2022-02-30 2022-04-31 2022-06-31 2022-11-31 2022-03-32 Don\'t match serial numbers: 2015-12-3135-89 '; $subst = "$1\"[[$3-$4]]\"$18"; $result = preg_replace($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