RegEx for CSS classes that does not select all periods

My regular expression `cssClassId` also selects dots in URLs and decimals for numbers. I could use some help selecting only the text after the dot for classes and not anywhere else. Check my current [regexr.com expression](https://regexr.com/76cea).

Start with the current cssClassId expression in the linked regexr.com example and compare its matches for CSS classes, URLs, and decimal numbers. Done means the expression selects text after dots only for class names and excludes dots elsewhere; verify the behavior against representative examples in the expression.

Written by the indexing model from the issue text.