Regex pattern for underscore or hyphen but not both

Question

I have a regular expression that is allowing a string to be standalone, separated by hyphen and underscore.

I need help so the string only takes hyphen or underscore, but not both.

This is what I have so far.

^([a-z][a-z0-9]*)([-_]{1}[a-z0-9]+)*$

foo             = passed
foo-bar         = passed
foo_bar         = passed
foo-bar-baz     = passed
foo_bar_baz     = passed
foo-bar_baz_qux = passed # but I don't want it to
foo_bar-baz-quz = passed # but I don't want it to

Answer 1

You may expand the pattern a bit and use a backreference to only match the same delimiter:

^[a-z][a-z0-9]*(?:([-_])[a-z0-9]+(?:\1[a-z0-9]+)*)?$

See the regex demo

Details:

• ^ - start of string
• [a-z][a-z0-9]* - a letter followed with 0+ lowercase letters or digits
• (?:([-_])[a-z0-9]+(?:\1[a-z0-9]+)*)? - an optional sequence of:
  • ([-_]) - Capture group 1 matching either - or _
  • [a-z0-9]+ - 1+ lowercase letters or digits
  • (?:\1[a-z0-9]+)* - 0+ sequences of:
    • \1 - the same value as in Group 1
    • [a-z0-9]+ - 1 or more lowercase letters or digits
• $ - end of string.