Regular expression with condition on inner expressions

I would like to build a regular expression for replacing a sentence with "per" when it should be (a readable version of a sentence with quantities).

That is:

• "3/unit" must match
• "unit/3" must match
• "feet/second" must match
• "05/07" must not match

I know how to create something like "\D+/\D+".
But how can I build a regex saying "not both right and left expressions match \D+" ?

>Solution :

You can use

^(?![0-9]+/[0-9]+$)[^/]+/[^/]+$

See the regex demo. Details:

• ^ – start of string
• (?![0-9]+/[0-9]+$) – a negative lookahead that fails the match if there are one or more digits, /, one or more digits and end of string position immediately to the right of the current location
• [^/]+/[^/]+ – one or more chars other than /, a / char, and then one or more chars other than /
• $ – end of string.