Regular Expressions.md

Regular Expressions

Basic Syntax

• /.../: Start and end regex delimiters
• |: Alternation
• (): Grouping

Position Matching

• ^: Start of string or start of line in multi-line mode
• \A: Start of string
• $: End of string or end of line in multi-line mode
• \Z: End of string
• \b: Word boundary
• \B: Not word boundary
• \<: Start of word
• \>: End of word

Character Classes

• \s: Whitespace
• \S: Not whitespace
• \w: Word
• \W: Not word
• \d: Digit
• \D: Not digit
• \x: Hexade­cimal digit
• \O: Octal digit

Special Characters

• \n: Newline
• \r: Carriage return
• \t: Tab
• \v: Vertical tab
• \f: Form feed
• \xxx: Octal character xxx
• \xhh: Hex character hh

Groups and Ranges

• .: Any character except newline (\n)
• (a|b): a or b
• (…): Group
• (?:…): Passive (non-c­apt­uring) group
• [abc]: a, b or c
• [^abc]: Not a, b or c
• [a-z]: Letters from a to z
• [A-Z]: Uppercase letters from A to Z
• [0-9]: Digits from 0 to 9

Note: Ranges are inclusive.

Quantifiers

• *: 0 or more
• +: 1 or more
• ?: 0 or 1
• {3}: Exactly 3
• {3,}: 3 or more
• {3,5}: 3, 4 or 5

Note: Quantifiers are greedy - they match as many times as possible. Add a ? after the quantifier to make it ungreedy.

Escape Sequences

• \:Escape following character. Used to escape any of the following metacharacters: {}^$.|*+?.
• \Q: Begin literal sequence
• \E: End literal sequence

String Replacement

• $1: 1st group
• $2: 2nd group
• $n: nth group
• `$``: Before matched string
• $': After matched string
• $+: Last matched string
• $&: Entire matched string

Note: Some regex implem­ent­ations use \ instead of $.

Assertions

• ?=: Lookahead assertion
• ?!: Negative lookahead
• ?<=: Lookbehind assertion
• ?!=, ?<!: Negative lookbehind
• ?>: Once-only subexp­ression
• ?(): Condition if-then
• ?()|: Condition if-then-else
• ?#: Comment

POSIX

• [:upper:]: Uppercase letters
• [:lower:]: Lowercase letters
• [:alpha:]: All letters
• [:alnum:]: Digits and letters
• [:digit:]: Digits
• [:xdigit:]: Hexade­cimal digits
• [:punct:]: Punctu­ation
• [:blank:]: Space and tab
• [:space:]: Blank characters
• [:cntrl:]: Control characters
• [:graph:]: Printed characters
• [:print:]: Printed characters and spaces
• [:word:]: Digits, letters and underscore

Pattern Modifiers

• g: Global match
• i: Case-i­nse­nsitive
• m: Multi-line mode. Causes ^ and $ to also match the start/end of lines.
• s: Single-line mode. Causes . to match all, including line breaks.
• x: Allow comments and whitespace in pattern
• e: Evaluate replac­ement
• U: Ungreedy mode