Regular Expressions 101

Pro

^start of line

Positive Lookahead(?=.{3,20}$)

.{3,20}any character (except for line terminators), repeated between 3 and 20 times

$end of line

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)

Group 1([_][a-z0-9]+)*

*repeats the group contents below any number of times:

Character Class[_]

_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

^(?=.{3,20}$)[a-z0-9]+([_][a-z0-9]+)*$

New

a_123
1_1
amit12343
12ami
a_457_h
a__a
a_______0_
_0_sd
0_0
12
a1
123456789012345678901