Regular Expressions 101

^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-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)

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

+repeats the group contents below at least once:

_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.

Search reference

Regular Expression

^(?=.{3,20}$)[a-zA-Z0-9]+(?:_[a-zA-Z0-9]+)+$

New

// should match
test_test_test_tests
test_test_test_tests
t_t_t_t_t_t_t_t_t_ts
t_t

// length 21
test_test_test_tests1
test_test_test_tests2
t_t_t_t_t_t_t_t_t_ts3

// length 2
t_

// underscore at start and end
test_test_test_test_
_est_test_test_tests

// double underscore
t_t_t_t_t__t_t_t_t_t

Regular Expression

Extraction