Given a LPeg grammar, I want to do a find/replace on some of the sub-matches.
For example, suppose that I want to replace all the occurrences of the letter "a" that are outside of double quotes. I came up with a grammar for that:
local re = require "re"
local pattern = re.compile([[
line <- (quoted / 'a' / .)*
quoted <- '"' rest
rest <- '"' / . rest
]])
It seems that re.gsub is not useful here, because it would mess with the parts inside quotes. The best I could come up with was to use a table capture, where we capture everything besides the "a". But it was unwieldy because the pattern returns a table instead of a string.
line <- {| ({quoted} / 'a' / {.})* |}
I also looked into substitution captures, but got stuck because I needed to add ->'%1' around everything other than the 'a'.
Turns out that the "%1" bit has to do with string captures, not of substitution captures. I can solve the problem by adding a capture only around the parts that I want to replace, and I can leave the other parts alone.
local re = require "re"
local pattern = re.compile([[
line <- {~ (quoted / 'a'->'' / .)* ~}
quoted <- '"' rest
rest <- '"' / . rest
]])
Pay attention to operator precedence, because '->' binds tightly
('a' 'b')->''