A Lua pattern question about optional patterns
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- Subject: A Lua pattern question about optional patterns
- From: Sean Conner <sean@...>
- Date: Mon, 9 Sep 2019 23:22:28 -0400
Normally I reach for LPEG to handle my parsing chores, but I have a
project where that ... well, I'd be reimplementing a form of regex anyway,
so why not use Lua patterns and avoid that mess.
But I have an issue that I do not know the answer to---I suspect there
isn't an answer but "use a real regex or LPEG". But I thought I would ask
anyway.
I have a pattern that looks like this BNF:
TEXT = 'A' - 'Z' / 'a' - 'z'
DIGIT = '0' - '9'
pattern = 1*TEXT [ ';' 1*DIGIT ]
In English, text, optionally followed by a semicolon and some digits. So
some valid examples:
foo
foo;1
foo;444
Invalid examples are
foo;
foo23
If there's a semicolon, it must be followed by a digit; if there's no
semicolon, no digits. This is trivial (to me) in LPEG. No so with Lua
patterns. There's:
"%a%;?(%d+)
but that allows "foo23" to slip through. What I would like would be:
%a(%;(%d+))?
although that doesn't work, since the '?' can (if I'm reading the
documentation right) only follow a character class, not a grouping.
Am I missing something?
-spc (I mean, besides "using LPEG"?)