Regular Expressions 101

^start of line

Non-Capturing Group(?:[a-zA-Z0-9]+[._]?[a-zA-Z0-9]+)+

+repeats the group contents below at least once:

Character Class[a-zA-Z0-9]+

+matches at least one character 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)

Character Class[._]?

?matches at most one character from the set below:

.the literal . (46₁₀ / 56₈ / 2E₁₆)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Character Class[a-zA-Z0-9]+

+matches at least one character 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)

$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

^(?:[a-zA-Z0-9]+[._]?[a-zA-Z0-9]+)+$

New

username
user_name
user.name
user.name_123
username$$$
_username
username_
user_.name
user._name
user__name
user..name

username
user_name
user.name
user.name_123
username$$$
_username
username_
user_.name
user._name
user__name
user..name