参考 - 这个正则表达式是什么意思? [英] Reference - What does this regex mean?
问题描述
这是什么?
这是一个常见问答集.这也是一个社区维基,所以请大家参与维护.
这是为什么?
freeformatter.com
xregexpregex.larsolavtorvik.com
php PCRE and POSIX, javascriptOffline:
- Microsoft Windows: RegexBuddy (analysis), RegexMagic (creation), Expresso (analysis, creation, free)
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?
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: php, perl, javascript, 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:
- 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.
Online tutorials
Quantifiers
- Zero-or-more:
*
:greedy,*?
:reluctant,*+
:possessive - One-or-more:
+
:greedy,+?
:reluctant,++
:possessive ?
:optional (zero-or-one)- 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+[[:alpha:]]
:POSIX character classes- 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
- Digit:
- Unicode categories (
\p{L}, \P{L}
, etc.)
Escape Sequences
- Horizontal whitespace:
\h
:space-or-tab,\t
:tab - Newlines:
- 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
^
:start of line/input,\b
:word boundary, and\B
:non-word boundary,$
:end of line/input\A
:start of input,\Z
:end of input php, perl, ruby\z
:the very end of input (\Z
in Python) .net, php, pcre, java, ruby, icu, swift, objective-c\G
:start of match php, perl, ruby
(Also see "Flavor-Specific Information → Java → The functions in Matcher
")
Groups
(...)
:capture group,(?:)
:non-capture group\1
:backreference and capture-group reference,$1
:capture group reference- What does a subpattern
(?i:regex)
mean? - What does the 'P' in
(?P<group_name>regexp)
mean? (?>)
:atomic group or independent group,(?|)
:branch reset- 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
- General named capturing group reference at
Lookarounds
- Lookaheads:
(?=...)
:positive,(?!...)
:negative - Lookbehinds:
(?<=...)
:positive,(?<!...)
:negative (not supported by javascript) - Lookbehind limits in:
- Lookbehind alternatives:
Modifiers
flag | modifier | flavors |
---|---|---|
a |
ASCII | python |
c |
current position | perl |
e |
expression | php perl |
g |
global | most |
i |
case-insensitive | most |
m |
multiline | php perl python javascript .net java |
m |
(non)multiline | ruby |
o |
once | perl ruby |
S |
study | php |
s |
single line | unsupported: javascript (workaround) | ruby |
U |
ungreedy | php r |
u |
unicode | most |
x |
whitespace-extended | most |
y |
sticky ↪ | javascript |
- How to convert preg_replace e to preg_replace_callback?
- What are inline modifiers?
- What is '?-mix' in a Ruby Regular Expression
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)
- php only:
- 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...
- 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
- 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
- "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?
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 -endfind()
): 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
- Official documentation:
- Boost regex engine: General syntax, Perl syntax (used by TextPad, Sublime Text, UltraEdit, ...???)
- JavaScript 1.5 general info and RegExp object
- PHP: pattern syntax,
preg_match
- Python: Regular expression operations,
search
vsmatch
, how-to - Rust: crate
regex
, structregex::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:
- 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
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
- Regex Pal, regular-expressions.info javascript
- 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 - Refiddle javascript ruby .net
Offline:
- Microsoft Windows: RegexBuddy (analysis), RegexMagic (creation), Expresso (analysis, creation, free)
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