Match PEG Pattern surrounded by the same pattern (LPEG)

Question

I'm trying to convert this regex pattern to lpeg: { *(?<!-)[, ](?!-) *} It's meant to be used as a splitting pattern, turning 2.5 2.6/2.5, 2.6 to 2.5 and 2.6, unless -'s are involved. It's part of identifying semver constraints.

I've used the split function from the lpeg docs, copied here

local function split (str, sep)
   sep = l.P(sep)
   local elem = l.C((1 - sep)^0)
   local p = l.Ct(elem * (sep * elem)^0)   -- make a table capture
   return p:match(str)
 end

My first stab at it works in the case there's a comma:

local h, sp = l.P'-', l.P' '
patt = sp^0 * (-h * l.S' ,' - h) * sp^0
split('2.5, 2.6', patt) -- works
split('2.5 2.6', patt) -- fails because the first sp^0 eats everything

However, it of course fails when there are only spaces. I cannot figure out how to maintain the original regex pattern's constraints and make it work in both cases. My other efforts have ended in infinite loops. Is there a way to approach capturing one instance of a pattern surrounded by greedy versions of itself, or some clever aspect of lpeg that can solve this problem?

Answer 1

The default matching method in LPeg is blind greedy matching, to simulate the behavior of your given regular expression, we need to construct a grammar with non-blind greedy matching,

S <- E1 S / E2

which will match multiple E1's followed by E2

local patt = l.P{
  's',
  s  = ' ' * l.V's' + l.V'suffix',
  suffix = -l.B'-' * l.S', ' * -l.P'-'  * l.P' '^0
}