Regular expression match for specific lower_case php validation [duplicate]

Question

I need to compose a regular expression for string, with a max length of 6 characters, containing only Latin letters in lowercase, with an optional underscore separator, without underscore starting and trailing.

I tried the following

^[a-z_]{1,6}$

But it allows underscore at the start and the end.

I also tried:

^([a-z]_?[a-z]){1,6}$

^(([a-z]+)_?([a-z]+)){1,6}$

^([a-z](?:_?)[a-z]){1,6}$

But nothing works. Please help.

Expecting:

Valid:

ex_bar

Not valid:

_exbar

exbar_

_test_

Answer 1

This is a fairly simple pattern that should work ^(?!_)[a-z_]{0,5}[a-z]$. See here for a breakdown.