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.

这是一个常见问答的集合，这也是一个社区维基，所以每个人都被邀请参与维护它。

Why is this?

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.

正则表达式正在遭受给我ZE代码类型的问题和没有解释的糟糕答案。此参考旨在提供指向质量问答的链接。

What's the scope?

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

此参考适用于以下语言：PHP、Perl、javascrip、python、ruby、Java、.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:

这可能过于宽泛，但这些语言共享相同的语法。对于特定的功能，后面有语言的标记，例如：

• What are regular expression Balancing Groups? .net

The Stack Overflow Regular Expressions FAQ

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

另请参见regex标记详细信息页面上的许多常规提示和有用的链接。

Online tutorials

在线教程

• RegexOne ↪


• Regular Expressions Info ↪

Quantifiers

量词

• 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

字符类别

• 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

转义序列

• 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

锚

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

“通用”指的是以下内容：icu Java javascrip.net Objective-c pcre Perl php python快速红宝石

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

* 默认|m多线模式。| D Dollar end only mode.

Groups

群组

• (...):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

环顾四周

• 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}
    
    
    
    
  
  
  
  

Modifiers

修饰词

• How to convert preg_replace e to preg_replace_callback?


• What are inline modifiers?


• What is '?-mix' in a Ruby Regular Expression

Other:

其他：

• |: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

常见任务

• Get a string between two curly braces: {...}


• Match (or replace) a pattern except in situations s1, s2, s3...


• How do I find all YouTube video ids in a string using a regex?


• 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

高级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
    
    
    
    
  
  
  
  

Flavor-Specific Information

特定口味的信息

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

(除标有*的链接外，此部分包含非堆栈溢出链接。)

• 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

一般信息

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

(标有*的链接是非堆栈溢出链接。)

• 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

可能导致正则表达式引擎失败的正则表达式示例

• Why does this regular expression kill the Java regex engine?

Tools: Testers and Explainers

工具：测试员和讲解员

(This section contains non-Stack Overflow links.)

(此部分包含非堆栈溢出链接。)

• Online (* includes replacement tester, + includes split tester):

  在线(*包括更换测试仪，+包括分离式测试仪)：

  
  
  
  
  • 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:

  脱机：

  
  
  
  
  • 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.)

  MySQL8.0：进行了各种语法更改。特别要注意的是，在某些情况下，反斜杠是双斜杠。(此答案需要进一步编辑，以反映差异。)