Reference - What does this regex mean ?.md

Reference - What does this regex mean?

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]

The list

Quantifiers:

• What does + mean?
• What does * mean?
• What does ? mean?
• What does {n, m} mean?
• Can someone explain Possessive Quantifiers to me? [tag:php], [tag:perl], [tag:java], [tag:ruby]
• What does ++ mean?
• What does *+ mean?
• Emulating possessive quantifiers [tag:.net]
• Difference between .*? and .* for regex: Greedy vs non-greedy, non-greedy also known as lazy, ungreedy or reluctant
• Greedy vs. Reluctant vs. Possessive Quantifiers

Character classes:

• How does a character class [] work?
• How does a negated character class [^] work?
• How does a substraction in character classes [range-[bar]] work? [tag:java], [tag:ruby] 1.9+
• How does intersection in character classes [range&&[bar]] work? [tag:java], [tag:ruby] 1.9+
• How does POSIX character classes [:alpha:] work?
• Why [^\\D2], [^[^0-9]2], [^2[^0-9]] get different results in Java? [tag:java]

Anchors:

• What does ^ mean?
• What does $ mean?
• What does \A mean? [tag:php], [tag:perl], [tag:python], [tag:ruby]
• What does \Z mean? [tag:php], [tag:perl], [tag:python], [tag:ruby]
• What does \G mean? [tag:php], [tag:perl], [tag:ruby]
• What does \b and \B mean? ^{word boundaries}

Groups:

• What does a capturing group () mean?
• What does a backreference \1 mean?
• What does a non capturing group (?:) mean?
• What does an atomic group (?>) mean?
• What does branch reset (?|) mean?
• How does named groups work ? ^{comment: since there are differences, we need to split this up}
• [tag:php] (?P<groupname>regex), (?<groupname>regex)
• [tag:python] (?P<groupname>regex)
• [tag:.net] ...
• [tag:perl] ...

Lookaround:

• What does a positive lookahead (?=) mean?
• What does a negative lookahead (?!) mean?
• What does a positive lookbehind (?<=) mean? Not supported by [tag:javascript]
• What does a negative lookbehind (?<!) mean? Not supported by [tag:javascript]
  • Lookbehind limits in: (something about fixed length)
    • [tag:php], [tag:perl], [tag:python], [tag:ruby]
    • [tag:java] (java is a bit more powerful AFAIK)
    • [tag:.net] has no limits?
  • Lookbehind alternatives:
    • Using \K [tag:php], [tag:perl]
      • Support of \K in regex
    • Alternative regex module for python [tag:python]
    • The hacky way
    • Mimicking Lookbehind in JavaScript ^{External link, someone got one on SO?}

Modifiers:

• What does the i modifier mean?
• What does the m modifier mean?
• What does the s modifier mean?
• What does the x modifier mean?
• What does the e modifier mean? [tag:php]
  • How to convert preg_replace e to preg_replace_callback?
• What are inline modifiers?

Escape sequence:

• What does \d mean?
• What does \D mean?
• What does \w mean?
• What does \W mean?
• What does \s mean?
• What does \S mean?
• What does \h mean?
• What does \H mean?
• What does \v mean?
• What does \V mean?
• What does \t mean?
• What does \r mean?
• What does \n mean?
• What does \e mean?
• What does \R mean? [tag:php]

Other: ^{not sure about the name ...}

• What does | mean?
• What does . mean?
• What special characters must be escaped?

Control verbes: [tag:php] [tag:perl]

• What does (*PRUNE) mean?
• What does (*SKIP) mean?
• What does (*FAIL)/(*F) mean?
• What does (*COMMIT) mean?
• What does (*MARK) mean?
• What does (*THEN) mean?
• What does (*ACCEPT) mean?

Recursion: [tag:php] [tag:perl]

• What does (?R) mean?
• What does (?0), (?1) mean?
• What does (?-1) mean?
• What does (?P>groupname)/(?&groupname) mean?

Advanced regex-fu

• match a string not containing a word
• How does this regex find triangular numbers?
• How to determine if a number is a prime with regex?
• How does this PCRE pattern detect palindromes?
• How can we match a^n b^n with Java regex?
• Match nested brackets
  • Using a recursive pattern [tag:php], [tag:perl]
  • Using balanced matching groups [tag:.net]
• “vertical” regex matching in an ASCII “image”
• Match strings whose length is a fourth power
• Verbs that act after backtracking and failure
• Match a number