Regular Expressions 101

Pro

^start of line

Negative Lookahead(?!_|\d)

1st Alternative_

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

2nd Alternative\d

\da digit

Negative Lookahead(?!.*__)

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

__the literal text __

Negative Lookahead(?!.*_$)

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

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

$end of line

\w+any word character, repeated at least once

$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

^(?!_|\d)(?!.*__)(?!.*_$)\w+$

New

_
A
abcd
_
123
some_spaces 
not_any_spaces__
1test34
correct_thing

_
A
abcd
_
123
some_spaces 
not_any_spaces__
1test34
correct_thing