When you create the regular expression for your custom entity, use the Python regular expression syntax. The following contains information about using Python regular expressions for custom entities.
Note: Regular expressions for custom entities are case insensitive.
Escape Backslash Characters
If your Python regular expression contains a backslash character, then you must replace each backslash character with two backslashes. For example, given a custom entity books\books, you would create the Python regular expression, books[\\]books.
Non-Capturing Groups
If you want to use parentheses () in your regular expression, use the non-capturing version (?:). Capturing contents of a group is not supported. Review the following example:
Correct - non-capturing version includes the question mark and colon (?:).
\s(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(?!\.)\bIncorrect - original version does not include the question mark and colon (?:).
\s((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(?!\.)\b