The Stack Overflow Regular Expressions FAQ

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

Online tutorials

• RegexOne ↪
• Regular Expressions Info ↪

Quantifiers

• Zero-or-more: *: greedy, *?: reluctant, *+: possessive
• One-or-more: +: greedy, +?: reluctant, ++: possessive
• Zero-or-one: ?: greedy, ??: reluctant, ?+: possessive
• Min/max ranges (all inclusive): {n,m}: between n & m, {n,}: n-or-more, {n}: exactly n
• Differences between greedy, reluctant (a.k.a. “lazy”, “ungreedy”) and possessive quantifier:
  • Greedy vs. Reluctant vs. Possessive Quantifiers
  • In-depth discussion on the differences between greedy versus non-greedy
  • What’s the difference between {n} and {n}?
  • Can someone explain Possessive Quantifiers to me? php, perl, java, ruby
  • Emulating possessive quantifiers .net
  • Non-Stack Overflow references: From Oracle, regular-expressions.info

Character Classes

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

Escape Sequences

• Horizontal whitespace: h:space-or-tab, t:tab
• Newlines:
  • r, n:carriage return and line feed
  • R:generic newline php java-8
• Negated whitespace sequences: H:Non horizontal whitespace character, V:Non vertical whitespace character, N:Non line feed character pcre php5 java-8
• Other: v:vertical tab, e:the escape character

Anchors

“Common” refers to the following: icu java javascript .net objective-c pcre perl php python swift ruby

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

Groups

• (...):capture group, (?:):non-capture group
  • How to capture multiple repeated groups?
  • Why is my repeating capturing group only capturing the last match?
• 1:backreference and capture-group reference, $1:capture group reference
  • What’s the meaning of a number after a backslash in a regular expression?
  • g<1>123:How to follow a numbered capture group, such as 1, with a number?: python
• What does a subpattern (?i:regex) mean?
• What does the ‘P’ in (?P<group_name>regexp) mean?
• (?>):atomic group or independent group, (?|):branch reset
  • Equivalent of branch reset in .NET/C# .net
• Named capture groups:
  • General named capturing group reference at regular-expressions.info
  • java: (?<groupname>regex): Overview and naming rules (Non-Stack Overflow links)
  • Other languages: (?P<groupname>regex) python, (?<groupname>regex) .net, (?<groupname>regex) perl, (?P<groupname>regex) and (?<groupname>regex) php
• (?<-foo>): balancing groups .net

Lookarounds

• Lookaheads: (?=...):positive, (?!...):negative
• Lookbehinds: (?<=...):positive, (?<!...):negative
• Lookbehind limits in:
  • Lookbehinds need to be constant-length php, perl, python, ruby
  • Lookarounds of limited length {0,n} java
  • Variable length lookbehinds are allowed .net
• Lookbehind alternatives:
  • Using K php, perl (Flavors that support K)
  • Alternative regex module for Python python
    • The hacky way
    • JavaScript negative lookbehind equivalents ^{External link}
• Tempered greedy token

Modifiers

• How to convert preg_replace e to preg_replace_callback?
• What are inline modifiers?
• What is ‘?-mix’ in a Ruby Regular Expression
• What does the caret ^ in (?^:…) mean in the string form of a Perl qr// Regex?

Other:

• |:alternation (OR) operator, .:any character, [.]:literal dot character
• What special characters must be escaped?
• Control verbs (php and perl): (*PRUNE), (*SKIP), (*FAIL) and (*F)
  • php only: (*BSR_ANYCRLF)
  • The (*SKIP)(*FAIL) idiom
• Recursion (php and perl): (?R), (?0) and (?1), (?-1), (?&groupname)

Common Tasks

• Get a string between two curly braces: {...}
• Match (or replace) a pattern except in situations s1, s2, s3…
• Find all YouTube video ids in a string
• Ignore middle part of a match
• Validation:
  • Internet: email addresses, URLs (host/port: regex and non-regex alternatives), passwords
  • Numeric: a number, min-max ranges (such as 1-31), phone numbers, date
  • Parsing HTML with regex: See “General Information > When not to use Regex”

Advanced Regex-Fu

• Strings and numbers:
  • Regular expression to match a line that doesn’t contain a word
  • How does this PCRE pattern detect palindromes?
  • Match strings whose length is a fourth power
  • How does this regex find triangular numbers?
  • How to determine if a number is a prime with regex?
  • How to match the middle character in a string with regex?
• Other:
  • How can we match a^n b^n?
  • Match nested brackets
    • Using a recursive pattern php, perl
    • Using balancing groups .net
  • “Vertical” regex matching in an ASCII “image”
  • List of highly up-voted regex questions on Code Golf
  • How to make two quantifiers repeat the same number of times?
  • An impossible-to-match regular expression: (?!a)a
  • Match/delete/replace this except in contexts A, B and C
  • Match nested brackets with regex without using recursion or balancing groups?
  • Can a Regex Return the Number of the Line where the Match is Found?
  • Matching overlapping strings
    • Manually modify the .lastIndex property javascript
    • With the third-party library regex python
  • Match a sequence of two distinct characters where the disparity in count is not greater than 3

Flavor-Specific Information

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

• Java
  • Official documentation: Pattern Javadoc ↪, Oracle’s regular expressions tutorial ↪
  • The differences between functions in java.util.regex.Matcher:
    • matches()): The match must be anchored to both input-start and -end
    • find()): A match may be anywhere in the input string (substrings)
    • lookingAt(): The match must be anchored to input-start only
    • (For anchors in general, see the section “Anchors”)
  • The only java.lang.String functions that accept regular expressions: matches(s), replaceAll(s,s), replaceFirst(s,s), split(s), split(s,i)
  • *An (opinionated and) detailed discussion of the disadvantages of and missing features in java.util.regex
• .NET
  • How to read a .NET regex with look-ahead, look-behind, capturing groups and back-references mixed together?
• Official documentation:
  • Boost regex engine: General syntax, Perl syntax (used by TextPad, Sublime Text, UltraEdit, …???)
  • JavaScript general info and RegExp object
  • .NET MySQL Oracle Perl5 version 18.2
  • PHP: pattern syntax, preg_match
  • Python: Regular expression operations, search vs match, how-to
  • Rust: crate regex, struct regex::Regex
  • Splunk: regex terminology and syntax and regex command
  • Tcl: regex syntax, manpage, regexp command
  • Visual Studio Find and Replace

General information

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

• Other general documentation resources: Learning Regular Expressions, *Regular-expressions.info, *Wikipedia entry, *RexEgg, Open-Directory Project
• DFA versus NFA
• Generating Strings matching regex
• Books: Jeffrey Friedl’s Mastering Regular Expressions
• 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. (blog post written by Stack Overflow’s founder)*
  • Do not use regex to parse HTML:
    • Don’t. Please, just don’t
    • Well, maybe…if you’re really determined (other answers in this question are also good)

Examples of regex that can cause regex engine to fail

• 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 (Also has a repository of useful regexes) javascript, python, pcre
  • *Regular Expressions 101 php, pcre, python, javascript, java, go, c#, rust
  • Regex Pal, regular-expressions.info javascript
  • Rubular ruby RegExr Regex Hero dotnet
  • *+ regexstorm.net .net
  • *RegexPlanet: Java java, Go go, Haskell haskell, JavaScript javascript, .NET dotnet, Perl perl php PCRE php, Python python, Ruby ruby, XRegExp xregexp
  • freeformatter.com xregexp
  • *+regex.larsolavtorvik.com php PCRE and POSIX, javascript

• Offline:

  • Microsoft Windows: RegexBuddy (analysis), RegexMagic (creation), Expresso (analysis, creation, free)

• MySQL 8.0: Various syntax changes were made. Note especially the doubling of backslashes in some contexts. (This Answer need further editing to reflect the differences.)