How to RegEx match double underscore, but not single ( __ not _)

Question

I'm working on a stylelint rule for modified BEM, which is essentially saying that a single underscore is not allowed, but a double underscore is.

match case:

• foo__bar

non-match cases:

• foo_bar

• foo_bar__baz

I would (ideally) like not just a working code snippet, but an explanation of how a problem like this is best solved using only JavaScript RegEx

Here are the relevant docs for stylelint: https://stylelint.io/user-guide/rules/selector-class-pattern/

Here is a test suite that would need to pass: https://regex101.com/r/NjwOa3/3/tests

Answer 1

You can try a regular expression like below.

^[^_]*_{2,2}[^_]*$

or try this for matching only the double underscores.

_{2,2}