Why does this regex for a comment Terminal not work?

[nchammas]

Here's a simple demo of what I'm trying to do.

from lark import Lark

parser = Lark(
    r"""
    start: closure
    closure: "(" ")"
    // Works:
    COMMENT: "#" /[^\n]/*
    // Doesn't work:
    // COMMENT: /#.*$/

    %import common.WS
    %ignore WS
    %ignore COMMENT
    """,
    start="start",
)

print(parser.parse(
    """
    ()

    # (
    """
))

I'm new to parsing. I started off with what I thought was the obvious way to write a Terminal for comments: COMMENT: /#.*$/

This definition is incidentally also used here in the docs.

It doesn't work, and the above test yields this error:

lark.exceptions.UnexpectedCharacters: No terminal matches '#' in the current parser context, at line 4 col 5

    # (
    ^

If I switch the Terminal definition to COMMENT: "#" /[^\n]/* (which I picked up from the docs here), it seems to work fine.

What's the difference between these two definitions? And why doesn't the "obvious" regex work?

There is a similar discussion over on #1281.

[MegaIng]

$ matches the end of the string by default. If you want it to match the end of the line, add the m flag. /#.*$/m. This is clearly documented in the python regex docs.

[nchammas]

OK, I see the documentation for m here.

So I suppose that, since . doesn't match newlines, I can even rewrite the Terminal as /#.*/, without the $.

(This is also how it's done in the Lark docs I referenced above--I just missed the lack of a $ at the end, which drove part of my confusion.)