Regular Expressions 101

Explanation
Pro

Group 1(^[a-z][a-z0-9._]+[a-z0-9]+$)

^start of line

Character Class[a-z]

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

Character Class[a-z0-9._]+

+matches at least one character from the set below:

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

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

.the literal . (46₁₀ / 56₈ / 2E₁₆)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Character Class[a-z0-9]+

+matches at least one character from the set below:

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

(^[a-z][a-z0-9._]+[a-z0-9]+$)

New

username123
USername12
user 12 
user-1234name$
user.name.
.username12
1username
username4444
user123user
user.name
user_name
user asdsad

username123
USername12
user 12 
user-1234name$
user.name.
.username12
1username
username4444
user123user
user.name
user_name
user asdsad