Reference - What does this regex mean? -

what this?

this collection of common q&a. community wiki, invited participate in maintaining it.

why this?

regex suffering give me ze code type of questions , poor answers no explanation. reference meant provide links quality q&a.

what's scope?

this reference meant following languages: php, perl, javascript, python, ruby, java, .net.

this might broad, these languages share same syntax. specific features there's tag of language behind it, example:

• what regular expression balancing groups? .net

the stack overflow regular expressions faq

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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") , possessive quantifier:
  • greedy vs. reluctant vs. possessive quantifiers
  • in-depth discussion on differences between greedy versus non-greedy
  • what's difference between {n} , {n}?
  • can explain possessive quantifiers me? php, perl, java, ruby
  • emulating possessive quantifiers .net
  • non-stack overflow references: oracle, regular-expressions.info

character classes

• what difference between square brackets , parentheses?
• [...]: 1 character, [^...]: negated/any character but
• [^] matches 1 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 [^\\d2], [^[^0-9]2], [^2[^0-9]] 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 , line feed
  • \r:generic newline php
• 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, , \b:non-word boundary, $:end of line/input
• \a:start of input, \z:end of input php, perl, python, ruby
• \g:start of match php, perl, ruby

(also see "flavor-specific information → java → functions in matcher")

groups

• (...):capture group, (?:):non-capture group
  • why repeating capturing group capturing last match?
• \1:backreference , capture-group reference, $1:capture group reference
  • \g<1>123:how follow numbered capture group, such \1, number?: python
• what subpattern (?i:regex) mean?
• what '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:
  • java: (?<groupname>regex): overview , naming rules (non-stack overflow links)
  • other languages: (?p<groupname>regex) python, (?<groupname>regex) .net, (?<groupname>regex) perl, (?p<groupname>regex) , (?<groupname>regex) php

lookarounds

• lookaheads: (?=...):positive, (?!...):negative
• lookbehinds: (?<=...):positive, (?<!...):negative (not supported javascript)
• lookbehind limits in:
  • lookbehinds need constant-length php, perl, python, ruby
  • lookarounds of limited length {0,n} java
  • variable length lookbehinds allowed .net
• lookbehind alternatives:
  • using \k php, perl (flavors support \k)
  • alternative regex module python python
    • the hacky way
    • javascript negative lookbehind equivalents ^{external link}

modifiers

• most flavors: g:global, i:case-insensitive, u:unicode, x:whitespace-extended
• c:current position perl e:expression php perl o:once ruby
• m:multiline php perl python javascript .net java, m:(non)multiline ruby
• s:single line (not supported javascript or ruby), s workaround javascript
• s:study php u:ungreedy php r
• how convert preg_replace e preg_replace_callback?
• what inline modifiers?
• what '?-mix' in ruby regular expression

other:

• |:alternation (or) operator, .:any character, [.]:literal dot character
• what special characters must escaped?
• control verbs (php , perl): (*prune), (*skip), (*fail) , (*f)
  • php only: (*bsr_anycrlf)
• recursion (php , perl): (?r), (?0) , (?1), (?-1), (?&groupname)

common tasks

• get string between 2 curly braces: {...}
• match (or replace) pattern except in situations s1, s2, s3...
• how find youtube video ids in string using regex?
• validation:
  • internet: email addresses, urls (host/port: regex , non-regex alternatives), passwords
  • numeric: a number, min-max ranges (such 1-31), phone numbers, date
  • parsing html regex: see "general information > when not use regex"

advanced regex-fu

• strings , numbers:
  • regular expression match line doesn't contain word?
  • how pcre pattern detect palindromes?
  • match strings length fourth power
  • how regex find triangular numbers?
  • how determine if number prime regex?
  • how match middle character in string regex?
• other:
  • how can match a^n b^n java regex?
  • match nested brackets
    • using recursive pattern php, perl
    • using balancing groups .net
  • “vertical” regex matching in ascii “image”
  • list of highly up-voted regex questions on code golf
  • how make 2 quantifiers repeat same number of times?
  • an impossible-to-match regular expression: (?!a)a
  • match/delete/replace this except in contexts a, b , c
  • match nested brackets regex without using recursion or balancing groups?

flavor-specific information

(except marked *, 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()): match must anchored both input-start , -end
    • find()): match may anywhere in input string (substrings)
    • lookingat(): match must anchored input-start only
    • (for anchors in general, see section "anchors")
  • the java.lang.string functions accept regular expressions: matches(s), replaceall(s,s), replacefirst(s,s), split(s), split(s,i)
  • *an (opinionated and) detailed discussion of disadvantages of , missing features in java.util.regex
• .net
  • how read .net regex look-ahead, look-behind, capturing groups , back-references mixed together?
• official documentation:
  • boost regex engine: general syntax, perl syntax (used textpad, sublime text, ultraedit, ...???)
  • javascript 1.5 general info , regexp object
  • .net mysql oracle perl5 version 18.2
  • php: pattern syntax, preg_match
  • python: regular expression operations, search vs match, how-to
  • splunk: regex terminology , syntax , regex command
  • tcl: regex syntax, manpage, regexp command

general information

(links marked * 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 not use regular expressions:
  • some people, when confronted problem, think "i know, i'll use regular expressions." have 2 problems. (blog post written stack overflow's founder)*
  • do not use regex parse html:
    • don't. please, don't
    • well, maybe...if you're really determined (other answers in question good)

examples of regex can cause regex engine fail

• why regular expression kill java regex engine?

tools: testers , explainers

(this section contains non-stack overflow links.)

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

  • debuggex (also has repository of useful regexes) javascript, python, pcre
  • *regular expressions 101 php, pcre, python, javascript
  • 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 , posix, javascript
  • refiddle javascript ruby .net

• offline:

  • microsoft windows: regexbuddy (analysis), regexmagic (creation), expresso (analysis, creation, free)