Regex not to allow double underscores

Question

Trying to apply regex for not allowing a string with double underscores

 [a-z][a-z0-9_-]+[^__]

but its failing in many cases like:

Answer 1

[^__] matches one character that is not underscore. To assert that your string doesn't have two consecutive underscores, you could use a negative lookahead:

^(?!.*__.*)[a-z][a-z0-9_-]+$

The lookaround asserts that your string does not have two consecutive underscores (?!.*__.*), and then matches your required string if the assertion does not fail -- [a-z][a-z0-9_-]+.