The hardest thing about writing a parser is cognitively accepting what is going to be considered valid input. You can make the best parser that is fast and well specified but invariably someone will (ab)use it in an unexpected way.

Famous examples: despite so many initial good intentions, html tags don’t need to be closed, JSON numbers are too often encoded as strings, YAML can look like what most people expect or it can look progressively more like JSON… and on and on.

If you draw a line from 'ad-hoc byte-wrangling nonsense' to 'parser combinators', this can't be more than 20% along it.

Looking at the linked URL parser, why doesn't it look like

  url = do scheme
           authority
           path
           query
           fragment
  where
  scheme = ...
  authority = ...
  etc.

It looks totally ad-hoc.

Unfortunately, simple URL parsing breaks on so many things. There is a reason on why every URL parsing library is at least a few thousand LOCs.

One common way to test it is just to pass ipv6 url: http://[f021:d981:b487:e57d:193e:550e::]/

  ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?

      scheme    = $2
      authority = $4
      path      = $5
      query     = $7
      fragment  = $9

  $2 (scheme) = http
  $4 (authority) = [f021:d981:b487:e57d:193e:550e::]
  $5 (path) = /

> LineReader splits input into lines, handles \n and \r\n, and trims the stray trailing \r that malformed input likes to leave behind

Is there a common source of extra \r in malformed inputs, beyond those existing as part of \r\n? Or is this just a dig at Windows-style line endings? If there's something weird going on I think I'd rather fail loudly.

> Bounding the inner scanner to a single line makes “run past the end of a malformed line” unrepresentable rather than merely unlikely.

I don't really see what makes it "unrepresentable", and this reads more like "if you used the right scanning logic, you can't have used the wrong scanning logic".

This post doesn't touch on something that makes parsers complicated no matter how simple the grammar: good error messages. Parsing a well formed input is the easy part, but not just spitting out a byte index but actually telling the user why their input is not good and what they could do to make it conform is super hard.

The Rust compiler is a common example of a compiler that does a good job here, and I think it is one of only a few.

> if (!line.accept('[').isEmpty() ) // [section] header.

Is this really ergonomic?

I now use nom to write my parsers. Once you understand it, it’s simple and parsing complex data becomes a _fun_ puzzle. I recommend it.

https://github.com/rust-bakery/nom

I'm going to collect this post after 24 hours, extract the methodologies from everyone's comments, and write them down in my notes. The reason I like HN is that people freely share their tips in the comments

If you created a format that is so difficult to parse that it cannot be parsed with simple readable C code then the problem is the format not the parser code.