Crystal regex tester

`[abc]`	a, b, or c
`[^abc]`	any character except a, b, or c
`[a-z]`	any character from range a-z
`[a-z0-9]`	any character from range a-z or 0-9
`.`	any character
`(...)`	group capture
`(a\|b)`	a or b (with capture)
`(?:a\|b)`	a or b (without capture)
`a?`	zero or one of a
`a*`	zero or more of a
`a+`	one or more of a
`a{3}`	exactly 3 of a
`a{3,}`	3 or more of a
`a{3,6}`	between 3 and 6 of a
`\s`	whitespace character
`\S`	non-whitespace character
`\d`	digit
`\D`	non-digit
`\w`	word character (letter, number, underscore)
`\W`	non-word character
`^`	beginning of line
`$`	end of line
`\A`	beginning of string
`\z`	end of string
`\Z`	end of string (matches just before newline if string ends with a newline)

`\b`	word boundary
`\B`	non-word boundary
`(?:...)`	non-capturing group
`(?<foo>...)`	named capture
`(?=abc)`	positive lookahead: ensures that the following characters match abc, but doesn't include those characters in the matched text
`(?!abc)`	negative lookahead: ensures that the following characters do not match abc, but doesn't include those characters in the matched text
`(?<=abc)`	positive lookbehind: ensures that the preceding characters match abc, but doesn't include those characters in the matched text
`(?<!abc)`	negative lookbehind: ensures that the preceding characters do not match abc, but doesn't include those characters in the matched text
`\1`	use the result of group capture 1
`\k<foo>`	use the result of named group capture foo
`(?1)`	indexed subroutine (repeat the pattern of group 1)
`(?&foo)`	named subroutine (repeat the pattern of group named foo)
`(?(DEFINE)...)`	predefined subroutines
`(?#...)`	comment

Reference