Reference - What does this regex mean?

Question

question from:https://stackoverflow.com/questions/65943236/extract-first-3-words-from-each-sentence-python

Answer 1

The Stack Overflow Regular Expressions FAQ

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

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Online tutorials

• RegexOne ?
• Regular Expressions Info ?

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
• Unicode categories (p{L}, P{L}, etc.)

Escape Sequences

• Horizontal whitespace: h:space-or-tab, :tab
• Newlines:
  • , :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

• ^:start of line/input, :word boundary, and B:non-word boundary, $:end of line/input
• A:start of input, :end of input php, perl, ruby
• z:the very end of input ( 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
  • 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 <a href="https://d