Question

I am trying to get a string to have an underscore ‘_’ everywhere on a case changes in a string to make it more clear to the user. For example if we have a String ‘personal IDnumber’, I want to make it to ‘personal_ID_number’. This is in C#.

Thank you, I appreciate any help, suggestions.

-Sid

Answer 1

Replace on a case change
You are looking for this (I removed the space before ID, assuming it a typo)

(?:(?<=[a-z])(?=[A-Z]))|(?:(?<=[A-Z])(?=[a-z]))

It will transform personalIDnumber into personal_ID_number

See it here on Regexr

the Lookbehind ((?<=[a-z])) and lookahead ((?=[A-Z])) construction is matching the empty string between a lower case and a uppercase letter (or the other way round in the second part after the pipe |) and replace this with a underscore.

Replace on a case change with optional whitespace
If you want to include whitespace in the replace, just include it between the lookarounds

(?:(?<=[a-z])\s*(?=[A-Z]))|(?:(?<=[A-Z])\s*(?=[a-z]))

It will transform personal IDnumber into personal_ID_number

See this here on Regexr

Replace on a case change, but each word starts with an uppercase
If you say every word starts with an uppercase letter, then you can do this

(?:(?<=[a-z])(?=[A-Z]))|(?:(?<=[A-Z])(?=[A-Z][a-z]))

This will make from

PersonalIDNumber
JustAnotherTest
OtherWordWithID
SomeMORETest

this

Personal_ID_Number
Just_Another_Test
Other_Word_With_ID
Some_MORE_Test

See this here on Regexr