#!/bin/sh

echo '
Character classes
  . any character except newline
  \w \d \s  word, digit, whitespace
  \W \D \S  not word, digit, whitespace
  [abc] any of a, b, or c
  [^abc]  not a, b, or c
  [a-g] character between a & g
Anchors
  ^abc$ start / end of the string
  \\b \\B word, not-word boundary
Escaped characters
  \. \* \\  escaped special characters
  \\t \\n \\r  tab, linefeed, carriage return
  \u00A9  unicode escaped ©
Groups and Lookaround
  (abc) capture group
  \\1  backreference to group #1
  (?:abc) non-capturing group
  (?=abc) positive lookahead
  (?!abc) negative lookahead
Quantifiers & Alternation
  a* a+ a?  0 or more, 1 or more, 0 or 1
  a{5} a{2,}  exactly five, two or more
  a{1,3}  between one & three
  a+? a{2,}?  match as few as possible
  ab|cd match ab or cd
'