Regex delimiter allow list but must all be the same

Is there a regex pattern that will have a list of allowed delimiters but enforce they all must be the same?

For an example, I will use a MAC Address:

xx:xx:xx:xx:xx:xx (allowed)

xx-xx-xx-xx-xx-xx (allowed)

xx:xx-xx:xx-xx:xx (not allowed)

I am currently using this:

[RegularExpression("^([0-9A-Fa-f]{2}[:-]){5}([0-9A-Fa-f]{2}){1}$")]

>Solution :

You can use a capture group and a backreference:

^[0-9A-Fa-f]{2}(?=([:-]))(?:\1[0-9A-Fa-f]{2}){5}$

• ^ Start of string
• [0-9A-Fa-f]{2} Repeat the character class 2 times
• (?=([:-])) Positive lookahead, assert and capture either : or -
• (?:\1[0-9A-Fa-f]{2}){5} Repeat 5 times a backreference \1 to what is initially captured in group 1 followed by 2 times the character class
• $ End of string

Regex demo

Or with an inline modifier for case insensitive if supported:

(?i)^[0-9a-f]{2}(?=([:-]))(?:\1[0-9a-f]{2}){5}$