The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

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Saturday, Debtember 19, 2020

LPEG vs. PEG—they both have their strengths and weaknesses

While the C PEG library is faster and uses less memory than LPEG, I still prefer using LPEG, because it's so much easier to use than the C PEG library. Yes, there's a learning curve to using LPEG, but its re module uses a similar syntax to the C PEG library, and it's easier to read and write when starting out. Another difference is that LPEG practically requires all the input to parse as a single string, whereas the C PEG can do that, it can also read data from a file (you can stream data to LPEG, but it involves more work—check out the difference between a JSON parser that takes the entire input as a string versus a JSON parser that can stream data; the later is nearly twice the size of the former).

The code isn't that much different. Here's a simple LPEG parser that will parse text like “” (a silly but simple example):

local re   = require "re"

return re.compile(
    tumbler <- number ( '.' number)*
    number  <- [0-9]+ -> pnum
    pnum = function(c) print(">>> " .. c) end,

Not bad. And here's the C PEG version:

tumbler <- number ('.' number)*
number	<- < [0-9]+ > { printf(">>> %*s\n",yyleng,yytext); }

Again, not terrible and similar to the LPEG version.

The major difference between the two, however, is in their use. In the LPEG version, tumbler can be used in other LPEG expressions. If I needed to parse something like “; charset=utf-8”, I can do that:

local re = require "re"

return re.compile(
    example <- %tumbler SP* ':' SP* %mimetype
    SP      <- ' ' / '\t'
    tumbler  = require "tumbler",
    mimetype = require "org.conman.parsers.mimetype",

The same cannot be said for the C PEG version. It's just not written to support such use. If I need to parse text like “” and mimetypes, then I have to modify the parser to support it all—there's no easy way to combine different parsers.

That said, I would still use the C PEG library, but only when memory or performance is an issue. It certainly won't be because of convenience.

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