Reference - What does this regex mean?

Regex Problem Overview

What is this?

This is a collection of common Q&A. This is also a Community Wiki, so everyone is invited to participate in maintaining it.

Why is this?

[tag:regex] is suffering from give me ze code type of questions and poor answers with no explanation. This reference is meant to provide links to quality Q&A.

What's the scope?

This reference is meant for the following languages: [tag:php], [tag:perl], [tag:javascript], [tag:python], [tag:ruby], [tag:java], [tag:.net].

This might be too broad, but these languages share the same syntax. For specific features there's the tag of the language behind it, example:

What are regular expression Balancing Groups? [tag:.net]

Regex Solutions

Solution 1 - Regex

The Stack Overflow Regular Expressions FAQ

See also a lot of general hints and useful links at the [tag:regex] tag details page.

Online tutorials

Quantifiers

Zero-or-more: [*:greedy][b1], *?:reluctant, [*+:possessive][b2]
One-or-more: [+:greedy][b1], +?:reluctant, [++:possessive][b3]
[?:optional (zero-or-one)][b4]
Min/max ranges (all inclusive): [{n,m}:between n & m][b5], [{n,}:n-or-more][b6], [{n}:exactly n][b7]
Differences between greedy, reluctant (a.k.a. "lazy", "ungreedy") and possessive quantifier:
- [Greedy vs. Reluctant vs. Possessive Quantifiers][b8]
- [In-depth discussion on the differences between greedy versus non-greedy][b9]
- [What's the difference between {n} and {n}?][b10]
- [Can someone explain Possessive Quantifiers to me?][b11] [tag:php], [tag:perl], [tag:java], [tag:ruby]
- [Emulating possessive quantifiers][b12] [tag:.net]
- Non-Stack Overflow references: From [Oracle][b13], [regular-expressions.info][b14]

Character Classes

[What is the difference between square brackets and parentheses?][c1]
[...]: any one character, [^...]: negated/any character but
[^] matches any one character including newlines [tag:javascript]
[\w-[\d]] / [a-z-[qz]]: set subtraction [tag:.net], [tag:xml-schema], [tag:xpath], JGSoft
[\w&&[^\d]]: set intersection [tag:java], [tag:ruby] 1.9+
[[:alpha:]]:POSIX character classes
[[:<:]] and [[:>:]] Word boundaries
Why do [^\\D2], [^[^0-9]2], [^2[^0-9]] get different results in Java? [tag:java]
Shorthand:
- Digit: [\d:digit][c2], [\D:non-digit][c3]
- Word character (Letter, digit, underscore): [\w:word character][c4], [\W:non-word character][c3]
- Whitespace: [\s:whitespace][c5], [\S:non-whitespace][c3]
Unicode categories (\p{L}, \P{L}, etc.)

Escape Sequences

Horizontal whitespace: [\h:space-or-tab][d1], [\t:tab][d2]
Newlines:
- [\r, \n:carriage return and line feed][d3]
- [\R:generic newline][d4] [tag:php] [tag:java-8]
Negated whitespace sequences: [\H:Non horizontal whitespace character, \V:Non vertical whitespace character, \N:Non line feed character][d5] [tag:pcre] [tag:php5] [tag:java-8]
Other: [\v:vertical tab][d6], [\e:the escape character][d7]

Anchors

anchor	matches	flavors
`^`	Start of string	Common*
`^`	Start of line	Common^m
`$`	End of line	Common^m
`$`	End of text	Common*
`$`	The very end of string	[tag:php]^D, [tag:javascript]
`\A`	Start of string	Common except [tag:js]
`\Z`	End of text	Common except [tag:js] [tag:python]
`\Z`	The very end of string	[tag:python]
`\z`	The very end of string	Common except [tag:js] [tag:python]
`\b`	Word boundary	Common
`\B`	Not a word boundary	Common
`\G`	End of previous match	Common except [tag:js], [tag:python] `re`

Term	Definition
[Start of string][a1]	At the very start of the string.
[Start of line][a2]	At the very start of the string, and after a non-terminal line terminator.
[End of string][a3]	At the very end of the string.
[End of text][a4]	At the very end of the string, and at a terminal line terminator.
[End of line][a5]	At the very end of the string, and at a line terminator.
[Word boundary][a6]	At a word character not preceded by a word character, and at a non-word character not preceded by a non-word character.
[End of previous match][a7]	At a previously set position, usually where a previous match ended. At the very start of the string if no position was set.

"Common" refers to the following: [tag:icu] [tag:java] [tag:js] [tag:.net] [tag:objective-c] [tag:pcre] [tag:perl] [tag:php] [tag:python] [tag:swift] [tag:ruby]

* Default [|] ^m Multi-line mode. [|] ^D Dollar end only mode.

Groups

[(...):capture group][f1], [(?:):non-capture group][f2]
- [Why is my repeating capturing group only capturing the last match?][f3]
[\1:backreference and capture-group reference, $1:capture group reference][f1]
- [What's the meaning of a number after a backslash in a regular expression?][f14]
- [\g<1>123:How to follow a numbered capture group, such as \1, with a number?:][f4] [tag:python]
[What does a subpattern (?i:regex) mean?][f5]
[What does the 'P' in (?P<group_name>regexp) mean?][f5a]
[(?>):atomic group][f6] or independent group, [(?|):branch reset][f7]
- [Equivalent of branch reset in .NET/C#][f8] [tag:.net]
Named capture groups:
- General named capturing group reference at regular-expressions.info
- [tag:java]: (?<groupname>regex): [Overview][f9] and [naming rules][f10] (Non-Stack Overflow links)
- Other languages: [(?P<groupname>regex)][f11] [tag:python], [(?<groupname>regex)][f12] [tag:.net], [(?<groupname>regex)][f13] [tag:perl], (?P<groupname>regex) and (?<groupname>regex) [tag:php]

Lookarounds

Lookaheads: [(?=...):positive][g1], [(?!...):negative][g2]
Lookbehinds: [(?<=...):positive][g3], [(?<!...):negative][g3]
Lookbehind limits in:
- [Lookbehinds need to be constant-length][g4] [tag:php], [tag:perl], [tag:python], [tag:ruby]
- [Lookarounds of limited length {0,n}][g5] [tag:java]
- [Variable length lookbehinds are allowed][g5] [tag:.net]
Lookbehind alternatives:
- [Using \K][g6] [tag:php], [tag:perl] ([Flavors that support \K][g7])
- [Alternative regex module for Python][g8] [tag:python]
  - [The hacky way][g9]
  - [JavaScript negative lookbehind equivalents][g10] ^{[External link][g11]}

Modifiers

flag	modifier	flavors
`a`	[ASCII][h17]	[tag:python]
`c`	[current position][h6]	[tag:perl]
`e`	[expression][h7]	[tag:php] [tag:perl]
`g`	[global][h1]	most
`i`	[case-insensitive][h2]	most
`m`	[multiline][h10]	[tag:php] [tag:perl] [tag:python] [tag:javascript] [tag:.net] [tag:java]
`m`	[(non)multiline][h9]	[tag:ruby]
`o`	[once][h8]	[tag:perl] [tag:ruby]
`S`	[study][h11]	[tag:php]
`s`	[single line][h12]	[tag:ruby]
`U`	[ungreedy][h4]	[tag:php] [tag:r]
`u`	[unicode][h3]	most
`x`	[whitespace-extended][h5]	most
`y`	[sticky ↪][h16]	[tag:javascript]

[How to convert preg_replace e to preg_replace_callback?][h14]
[What are inline modifiers?][h15]
What is '?-mix' in a Ruby Regular Expression

Other:

[|:alternation (OR) operator][i1], [.:any character][h12], [.]:literal dot character
[What special characters must be escaped?][i2]
Control verbs ([tag:php] and [tag:perl]): [(*PRUNE)][i3], [(*SKIP)][i3], [(*FAIL) and (*F)][i3]
- [tag:php] only: [(*BSR_ANYCRLF)][i4]
Recursion ([tag:php] and [tag:perl]): [(?R)][i5], [(?0) and (?1)][i6], [(?-1)][i7], [(?&groupname)][i8]

Common Tasks

[Get a string between two curly braces: {...}][j1]
[Match (or replace) a pattern except in situations s1, s2, s3...][j2]
https://stackoverflow.com/q/5830387
Validation:
- Internet: [email addresses][j3], [URLs][j4] (host/port: [regex][j5] and [non-regex][j6] alternatives), [passwords][j7]
- Numeric: [a number][j8], [min-max ranges (such as 1-31)][j9], [phone numbers][j10], [date][j11]
- Parsing HTML with regex: See "General Information > When not to use Regex"

Advanced Regex-Fu

Strings and numbers:
- https://stackoverflow.com/q/406230
- https://stackoverflow.com/q/3746487
- [Match strings whose length is a fourth power][k1]
- https://stackoverflow.com/q/3627681
- https://stackoverflow.com/q/2795065
- How to match the middle character in a string with regex?
Other:
- https://stackoverflow.com/q/3644266
- Match nested brackets
  - [Using a recursive pattern][k7] [tag:php], [tag:perl]
  - [Using balancing groups][k8] [tag:.net]
- “Vertical” regex matching in an ASCII “image”
- [List of highly up-voted regex questions on Code Golf][k4]
- How to make two quantifiers repeat the same number of times?
- [An impossible-to-match regular expression: (?!a)a][k5]
- [Match/delete/replace this except in contexts A, B and C][k6]
- [Match nested brackets with regex without using recursion or balancing groups?][k9]

Flavor-Specific Information

(Except for those marked with *, this section contains non-Stack Overflow links.)

Java
- Official documentation: [Pattern Javadoc ↪][l1], [Oracle's regular expressions tutorial ↪][l2]
- The differences between functions in [java.util.regex.Matcher][l3]:
  - [matches()][l4]): The match must be anchored to both input-start and -end
  - [find()][l5]): A match may be anywhere in the input string (substrings)
  - [lookingAt()][l6]: The match must be anchored to input-start only
  - (For anchors in general, see the section "Anchors")
- The only [java.lang.String][l7] functions that accept regular expressions: [matches(s)][l8], [replaceAll(s,s)][l9], [replaceFirst(s,s)][l10], [split(s)][l11], [split(s,i)][l13]
- *[An (opinionated and) detailed discussion of the disadvantages of and missing features in java.util.regex][l14]
.NET
- [How to read a .NET regex with look-ahead, look-behind, capturing groups and back-references mixed together?][l31]
Official documentation:
- Boost regex engine: [General syntax][l14], [Perl syntax][l15] (used by TextPad, Sublime Text, UltraEdit, ...???)
- JavaScript [general info][l16] and [RegExp object][l17]
- [.NET][l18] ![][|] [MySQL][l19] ![][|] [Oracle][l20] ![][|] [Perl5 version 18.2][l21]
- PHP: [pattern syntax][l22], [preg_match][l23]
- Python: [Regular expression operations][l24], [search vs match][l25], [how-to][l26]
- Rust: [crate regex][132], [struct regex::Regex][133]
- Splunk: [regex terminology and syntax][l27] and [regex command][l28]
- Tcl: [regex syntax][tclsyntax], [manpage][l29], [regexp command][l30]
- Visual Studio Find and Replace

General information

(Links marked with * are non-Stack Overflow links.)

Other general documentation resources: https://stackoverflow.com/q/4736, *[Regular-expressions.info][m1], *[Wikipedia entry][m2], *[RexEgg][m3], [Open-Directory Project][m4]
[DFA versus NFA][m5]
Generating Strings matching regex
Books: Jeffrey Friedl's [Mastering Regular Expressions][m6]
When to not use regular expressions:
- [Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.][m7] (blog post written by [Stack Overflow][so]'s founder)*
- Do not use regex to parse HTML:
  - [Don't][m8]. ![][|] [Please, just don't][m9]
  - [Well, maybe...if you're really determined][m10] (other answers in this question are also good)

Examples of regex that can cause regex engine to fail

https://stackoverflow.com/questions/288800/why-does-this-regular-expression-kill-the-java-regex-engine

Tools: Testers and Explainers

(This section contains non-Stack Overflow links.)

Online (* includes replacement tester, + includes split tester):
- [Debuggex][n1] (Also has a repository of useful regexes) [tag:javascript], [tag:python], [tag:pcre]
- *[Regular Expressions 101][n2] [tag:php], [tag:pcre], [tag:python], [tag:javascript], [tag:java]
- [Regex Pal][n3], [regular-expressions.info][n4] [tag:javascript]
- [Rubular][n5] [tag:ruby] ![][|] [RegExr][n6] ![][|] [Regex Hero][n7] [tag:dotnet]
- *+ [regexstorm.net][n8] [tag:.net]
- *RegexPlanet: [Java][n9] [tag:java], [Go][n10] [tag:go], [Haskell][n11] [tag:haskell], [JavaScript][n12] [tag:javascript], [.NET][n13] [tag:dotnet], [Perl][n14] [tag:perl] [tag:php] [PCRE][n15] [tag:php], [Python][n16] [tag:python], [Ruby][n17] [tag:ruby], [XRegExp][n18] [tag:xregexp]
- [freeformatter.com][n19] [tag:xregexp]
- *+[regex.larsolavtorvik.com][n20] [tag:php] PCRE and POSIX, [tag:javascript]
Offline:
- Microsoft Windows: [RegexBuddy][o1] (analysis), [RegexMagic][o2] (creation), [Expresso][o3] (analysis, creation, free)

Content Type	Original Author	Original Content on Stackoverflow
Question	HamZa	View Question on Stackoverflow
Solution 1 - Regex	aliteralmind	View Answer on Stackoverflow