Regular Expressions 101

Explanation

Pro

^start of line

Negative Lookahead(?!.*___|\.|.*\.$|.*\.\.)

1st Alternative.*___

.*any character (except for line terminators), repeated any number of times

___the literal text ___

2nd Alternative\.

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

3rd Alternative.*\.$

.*any character (except for line terminators), repeated any number of times

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

$end of line

4th Alternative.*\.\.

.*any character (except for line terminators), repeated any number of times

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[a-z1-9_.]{3,20}

{3,20}matches between 3 and 20 characters from the set below:

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

1-9one character from 1 (49₁₀) to 9 (57₁₀)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

.the literal . (46₁₀ / 56₈ / 2E₁₆)

$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

i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters

Detailed match information will be displayed here automatically.

25 tokens

^(?!.*___|\.|.*\.$|.*\.\.)[a-z1-9_.]{3,20}$

New

.test
_test
_.test
__test
test_
te_.st
te.st
..test

.test
_test
_.test
__test
test_
te_.st
te.st
..test