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Need help to understand LPeg and PEGs

Tags:

lua

peg

lpeg

The following pattern (from this page) matches only strings with balanced parentheses:

b = lpeg.P{ "(" * ((1 - lpeg.S"()") + lpeg.V(1))^0 * ")" }

What does 1- in 1 - lpeg.S"()" mean?

function gsub (s, patt, repl)
  patt = lpeg.P(patt)
  patt = lpeg.Cs((patt / repl + 1)^0)
  return lpeg.match(patt, s)
end

What does the +1 in patt / repl + 1 mean?

And I still not quite get the function of prioritized choice operator / very well from this paper

Any help will be appreciated!

like image 989
Decula Avatar asked Oct 18 '13 21:10

Decula


1 Answers

The 1 in 1 - lpeg.S"()" means any character. The whole statement can be read as, match any character while not matching a character in the set "()".

The +1 is the same idea, if repl is a string then patt / repl + 1 matches pattern patt and then replaces it's capture with the string repl or skips a character.

like image 92
Adam Avatar answered Nov 09 '22 02:11

Adam