Regular Expressions 101

^start of line

Negated Character Class[^\W_]

\Wany non-word character

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Negative Lookahead(?!.*?[._]{2})

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

Character Class[._]{2}

{2}matches exactly 2 characters from the set below:

.the literal . (46₁₀ / 56₈ / 2E₁₆)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Character Class[\w.]{6,18}

{6,18}matches between 6 and 18 characters from the set below:

\wany word character

.the literal . (46₁₀ / 56₈ / 2E₁₆)

Negated Character Class[^\W_]

\Wany non-word character

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

$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

Regular Expression

^[^\W_](?!.*?[._]{2})[\w.]{6,18}[^\W_]$

New

foo.1bar
foo._bar
_foo.1bar
foo.1bar.

foo.1bar
foo._bar
_foo.1bar
foo.1bar.