Ask HN: What is the simplest way for users to write custom parsers?

I'm currently working on some software targeted at mathematicians. Part of this requires that the user can write parsers to parse custom notation. I would expect most mathematicians interested in this project to know at least some basic programming but I also think it would be too difficult to expect them to write a full recursive descent parser or similar for their own notation. My current plan is to expose a simple parser combinator library alternatively I was thinking of a parser generator instead. But I was wondering if there is an option which is even simpler for users. What is the simplest way to allow users to write custom parsers?