Regular Expressions

Common regexes

purpose/match/validates	Regex
FQDN^1,²	`(?=^.{1,254}$)(^(?:(?!\d\|-)[a-zA-Z0-9\-]{1,63}(?<!-)\.?)+(?:[a-zA-Z]{2,})$)`

online tools

Regexr – evaluates and explains regular expressions
Regexpal – evaluates regexes
Regex101 – online RegEx IDE
Regex Crossword – game to learn regular expressions

^[1] fully qualified domain name

^[2] from http://blog.gnukai.com/2010/06/fqdn-regular-expression/ which references RFC1035