Run-time problem

P51 - Syntax error in regular expression

Most problems are detected when Inform translates the source text, but this is one of the exceptions, a "run-time problem" which emerges only during play.

Regular expressions are concise, usually finely-honed texts which use a special notation to specify what to search for when looking through other text. The notation is quite a difficult one to learn, and it's very easy even for experts to make mistakes by writing something which does not make sense - that is, to write a regular expression which is meaningless, and so can't be searched for. That's what seems to have happened here.