I'm getting the following error trying to integrate regex into a
transform that previously used concat and string searching as it's
much more flexible.

Your XSL file is not well formed so generates an error in the XML parser
you are missing a " to close your selct attribute here

format-number(regex-group(4), '00'), ':', regex-group(5), ':',
regex-group(6) />

After that the file is well formed so would be passed to teh XSLT engine
but would generate XSLt syntax errors as it has xsl:template inside
xsl:template, sorry I can't guess what was intended there so I can not
suggest a fix.