The fields on the "New failure cause" view have different validator behaviours. The "name" and "description" fields seem to validate their contents upon loss of focus (see JENKINS-47674), but the single and multi line pattern fields only validate upon submitting the form. This, however, leads to the loss of entered data if a validation error occurs.
On top of this, the displayed error message is helpfully arranged to highlight the exact character causing the syntax error, but the page is not configured to use monospace characters, therefore the pointer is usually way off.
Example (in non-monospace to show the problem):
Bad syntax! Illegal/unsupported escape sequence near index 52
.*java\.lang\.AssertionError: Execute exception: C:\N[REDACTED] ^