Regular Expressions 101

Explanation

Pro

^start of line

Negative Lookahead(?!_(?!_))

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Negative Lookahead(?!_)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Negative Lookahead(?!(?:.*[^_\n])?_$)

Non-Capturing Group(?:.*[^_\n])?

?repeats the group contents below at most once:

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

Negated Character Class[^_\n]

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

\na line-feed (newline) character (ASCII 10)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

$end of line

Character Class[\w.-]+

+matches at least one character from the set below:

\wany word character

.the literal . (46₁₀ / 56₈ / 2E₁₆)

-the literal - (45₁₀ / 55₈ / 2D₁₆)

$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

Match Information

Detailed match information will be displayed here automatically.

Quick Reference

26 tokens

Regular Expression

^(?!_(?!_))(?!(?:.*[^_\n])?_$)[\w.-]+$

New

Valid Entries: 
abc
ABC
Abc123
123a
Abc_12
Abc_12_3a
abc_23_43_fg
___should_work_but_doesnt__________
___ok__
___ok____
__a__

Invalid Entries: 
_abc
abc_
_abc_