Regular Expressions 101

Explanation

Pro

Definition Group(?(DEFINE)(?<label>([[:alpha:]]([[:alnum:]-]{0,61}[[:alnum:]])?)|\*))

Group label(?<label>([[:alpha:]]([[:alnum:]-]{0,61}[[:alnum:]])?)|\*)

1st Alternative([[:alpha:]]([[:alnum:]-]{0,61}[[:alnum:]])?)

Group 2([[:alpha:]]([[:alnum:]-]{0,61}[[:alnum:]])?)

Character Class[[:alpha:]]

[:alpha:]an alphabetic character [a-zA-Z]

Group 3([[:alnum:]-]{0,61}[[:alnum:]])?

?repeats the group contents below at most once:

Character Class[[:alnum:]-]{0,61}

{0,61}matches between 0 and 61 characters from the set below:

[:alnum:]an alphanumeric character [a-zA-Z0-9]

-the literal - (45₁₀ / 55₈ / 2D₁₆)

Character Class[[:alnum:]]

[:alnum:]an alphanumeric character [a-zA-Z0-9]

2nd Alternative\*

\*the literal * (42₁₀ / 52₈ / 2A₁₆)

Positive Lookahead(?=^.{1,255}$)

^start of line

.{1,255}any character (except for line terminators), repeated between 1 and 255 times

$end of line

Non-Capturing Group(?:^(?P<name>(?P>label))\.(?P<zone>(?:(?P>label)\.?)*)$)

^start of line

Group name(?P<name>(?P>label))

(?P>label)calls group label as a subroutine

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Group zone(?P<zone>(?:(?P>label)\.?)*)

Non-Capturing Group(?:(?P>label)\.?)*

*repeats the group contents below any number of times:

(?P>label)calls group label as a subroutine

\.?optionally matches the literal . (46₁₀ / 56₈ / 2E₁₆)

$end of line

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Detailed match information will be displayed here automatically.

25 tokens

(?(DEFINE)(?<label>([[:alpha:]]([[:alnum:]-]{0,61}[[:alnum:]])?)|\*))(?=^.{1,255}$)(?:^(?P<name>(?P>label))\.(?P<zone>(?:(?P>label)\.?)*)$)

New

google.com
mail.google.com
test-.google.com
*.google.com
google.com.
101.com

google.com
mail.google.com
test-.google.com
*.google.com
google.com.
101.com