Regular Expressions 101

Pro

Positive Lookahead(?=^.{1,254}$)

^start of line

.{1,254}any character (except for line terminators), repeated between 1 and 254 times

$end of line

Group 1(^(?>(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{2,})$)

^start of line

Atomic Group(?>(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+

+repeats the group contents below at least once:

Negative Lookahead(?!\d+\.)

\d+a digit, repeated at least once

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[a-zA-Z0-9_\-]{1,63}

{1,63}matches between 1 and 63 characters from the set below:

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

0-9one character from 0 (48₁₀) to 9 (57₁₀)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

\-the literal - (45₁₀ / 55₈ / 2D₁₆)

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

Non-Capturing Group(?:[a-zA-Z]{2,})

Character Class[a-zA-Z]{2,}

{2,}matches at least 2 characters from the set below:

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

$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

Regular Expression

(?=^.{1,254}$)(^(?>(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{2,})$)

New

Match:
host.domain.com
_host.domain.com
1host-2._ldap.domain.com

Non-Matches:
33.domain.com
host..com
this_really_long._fully_qualified_domain_name._contains_257_characters_w