RegEx Cheatsheet
https://www.debuggex.com/cheatsheet/regex/pcre
https://remram44.github.io/regex-cheatsheet/regex.html
http://whyjoseph.blogspot.com/2011/04/pcre-cheat-sheet-article-201102.html
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
Groups & 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