regex101: JavaScript Match Workspace

\Bnot an ASCII word boundary

@the literal @ (64₁₀ / 100₈ / 40₁₆)

Group 1([a-z0-9](?:-(?=[a-z0-9])|[a-z0-9]){0,38}(?<=[a-z0-9]))

Character Class[a-z0-9]

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

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Non-Capturing Group(?:-(?=[a-z0-9])|[a-z0-9]){0,38}

{0,38}repeats the group contents below between 0 and 38 times:

1st Alternative-(?=[a-z0-9])

-the literal - (45₁₀ / 55₈ / 2D₁₆)

Positive Lookahead(?=[a-z0-9])

Character Class[a-z0-9]

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

0-9one character from 0 (48₁₀) to 9 (57₁₀)

2nd Alternative[a-z0-9]

Character Class[a-z0-9]

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

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Positive Lookbehind(?<=[a-z0-9])

Character Class[a-z0-9]

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

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Global pattern flags

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

g modifier: global. Finds all matches instead of stopping after the first

26 tokens

Regular Expression

\B@([a-z0-9](?:-(?=[a-z0-9])|[a-z0-9]){0,38}(?<=[a-z0-9]))

New

@username

@user in a @user2 pagagraph of @user: text @usern-ame! @inalid-  

@d

@12345678912345678912345678911112345678904 

@invalid--username

and @valid-username-: l

@-a
@-invalid
@1
@a

@invalid-

hdj @hfd.com

some @valid-dd username

email@test.com 

@validu-sername- but is it worth it?

@johndoe.
@johndoe-
@johndoe-a

@fo-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o

Substitution

@username

@user in a @user2 pagagraph of @user: text @usern-ame! @inalid-  

@d

@12345678912345678912345678911112345678904 

@invalid--username

and @valid-username-: l

@-a
@-invalid
@1
@a

@invalid-

hdj @hfd.com

some @valid-dd username

email@test.com 

@validu-sername- but is it worth it?

@johndoe.
@johndoe-
@johndoe-a

@fo-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o