How could one write a parser for BNF without causing and infinite loop in the following case:

Something ::= AnotherThing|Something

?

What are you trying to parse? The form S ::= a | S can only parse a single "a". It's pretty much equivalent to S ::= a because S ::= S does nothing (might even be removed if your parser does any optimization during construction).

If your parser looks at the first match only (as opposed to longest match), this rule shouldn't be an issue.

@RememberMe Here's a more realistic example. Matching addition expressions

Addition ::= Number '+' (Number|Addition)
Number ::= 'regexp:[0-9]+'

This would allow you to match something like:

1+2+3+4+5

Without the Number OR (|) Expression it would only match 1+2, 3+4, which would also be separate matches.

@Berkmann18 okay I see. They way my parser works at the moment is that if it finds a reference to another rule it looks up that rule, which in this case would require a lookup of the same rule again, and again in infinity

BNF? This brings back memories.

@pythondev I see, are you able to get it to short-circuit by stopping at the first match?