A Lua pattern question about optional patterns

[Sean Conner <sean@...>]

  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"?)