PATINDEX returning 0 on matching rexpresson - regex

Question

As far as I found out, T-SQL has a possibility to check for regular expressions, via
PATINDEX.
My task is simple: I get a telecom-Type (phone for example). On a mapping table, a regex to validate this type is saved. So before saving, I'd like to check this regex.
So easy done:
-- Check the regular expression for phone
DECLARE #regExp nvarchar(255);
SELECT #regExp = tt.ValidationRegex
FROM Core.TelecomType tt
WHERE tt.Code = #DEFAULT_PHONE_TYPE;
Sadly, this always returns 0, even with wildcards with tries like this:
SET #regExp = CONCAT('%', #regExp, '%');
SET #regExp = CONCAT(#regExp, '%');
SET #regExp = CONCAT('%', #regExp );
On RegExr and oracle-side, the values seem to match, so is this a problem on the T-SQL? If yes, is there a workarround for this?
Thanks in advance
Matthias

Answer 1

Nope, sorry. PATINDEX doesn't let you match a regular expression, it works with the same kinds of patterns that are used with LIKE.
A quote from the documentation:
PATINDEX works just like LIKE, so you can use any of the wildcards. You do not have to enclose the pattern between percents. PATINDEX('a%', 'abc') returns 1 and PATINDEX('%a', 'cba') returns 3.
Unlike LIKE, PATINDEX returns a position, similar to what CHARINDEX does.
If you need regular expression matching in T-SQL, you'll have to rely on a custom CLR function/procedure. You can also check this to see if you can use it.