// REXPLORE

. any char (except \n)
\d digit [0-9]
\w word [a-zA-Z0-9_]
\s whitespace
\D \W \S negations
\b word boundary
^ start of string
$ end of string

* 0 or more
+ 1 or more
? 0 or 1
{n} exactly n
{n,} n or more
{n,m} n to m
*? +? ?? lazy versions

(abc) capture group
(?:abc) non-capture
(?=abc) lookahead
(?!abc) neg lookahead
(?<=abc) lookbehind
(?<!abc) neg lookbehind
(?<name>abc) named group

[abc] a, b, or c
[^abc] not a, b, c
[a-z] range a to z
[a-zA-Z] letter

a|b a or b
\ escape special