I want to reference a wildcard match and use that reference to populate a field.
For example:
CASE WHEN [Co1] LIKE 'User%' THEN [Col2] = [the respective User% matched earlier]
How can I do that?
I know how to do it in REGEX, where you use parenthesis in the wildcard to reference the match and then you use \1 to use that wildcard match.
T-SQL does not support regex. There are some watered-down commands you can use for wild-card matching etc. There is no feature that allows you to use back-references.
There have been attempts to implement regex support using the .Net framework via CLR - for example here. I have not experimented with any such implementation; you may want to review those.
Often times, people find work arounds for many of the common tasks, so it is possible that you can use the features within T-SQL to accomplish the task.