[SOLVED] How can I compose a parser to parse a quoted regex in fastparse?

How can I compose a parser to parse a quoted regex in fastparse?

What I wish to parse is any Regex that quoted with double quotes. For example, "([A-Z]+[A-Z]+[C])"

What I have tried so far is the following in Scala using fastparse library:

  def regex[_: P]: P[Unit] = P(AnyChar.rep).log
  def quotedRegex[_: P]: P[Unit] = P("\"" ~ regex ~ "\"").log

  val Parsed.Failure(label, index, extra) = parse(""""str"""", quotedRegex(_))

But this throws exception:

+quotedRegex:1:1, cut
  +regex:1:2, cut
  -regex:1:2:Success(1:6, cut)
-quotedRegex:1:1:Failure(quotedRegex:1:1 / "\"":1:6 ..."\"str\"", cut)
label = "\""
index = 5
trace = TracedFailure((any-character | "\""),(any-character | "\""),Parsed.Failure(Expected "\"":1:6, found ""))

What I understood so far is that regex parser is consuming the last double quote as well. But I am not able to figure out how to avoid that! I presume we need to write a lookahead of some sort and somehow avoid parsing the last character but not sure how to do this.

Please help.

Solution

To do negative lookaheads, use !. It will make sure the next character is not a double quote, but doesn't actually consume anything, just like a negative lookahead in normal regex. Then you can match on AnyChar or some other pattern.

def regex[_: P]: P[Unit] = P((!"\"" ~ AnyChar).rep).log

Here it is running in Scastie.