Matching something at the beginning of the first line

Lezer
1

Here is an example of the behavior that I would like to have for a simplified grammar:

# 1. One number.
100
==> Program(Number)

# 2. Two numbers.
100
200
==> Program(Number, Number)

# 3. Comment after space.
100 /Comment
200
==> Program(Number, LineComment, Number)

# 4. Comment after newline.
100
/Comment
200
==> Program(Number, LineComment, Number)

# 5. Comment after newline 2
100/200
/Comment
==> Program(Number, Adverb, Number, LineComment)

# 6. Comment at the start of the first line.
/Comment
200
==> Program(LineComment, Number)

Here is the grammar.

@top Program { sentences }
@skip { LineComment }
sentences { (sentence newline)* sentence }
sentence { Number | Number Adverb Number }
@tokens {
  @precedence { LineComment, newline }
  Adverb { "/" }
  Number { @digit+ }
  LineComment { " "+ "/" ![\n]* | "\n/" ![\n]* }
  space { " "+ }
  newline { "\n" }
}

Test case 6 doesn’t parse. I understand why, but I’m not sure how to fix it. What I would like to do is interpret / as the start of a comment if it’s at the beginning of a line or if it follows a space. I can match the beginnings of other lines by detecting a \n.

2

The easiest way to do this would be to use an external tokenizer that looks at the character before the token and only returns a comment token if it sees a slash and the thing before it was either the start of the input, a space, or a newline character.

3

Thanks, I tried that and it worked well.