package ppx_regexp

ppx_regexp - Matching Regular Expressions with OCaml Patterns

This syntax extension turns

match%pcre x with
| {|re1|} -> e1
...
| {|reN|} -> eN
| _ -> e0

into suitable invocations of the Re library. The patterns are plain strings of the form accepted by Re_pcre, with the following additions:

• (?<var>...) defines a group and binds whatever it matches as var. The type of var will be string if the match is guaranteed given that the whole pattern matches, and string option if the variable is bound to or nested below an optionally matched group.

• ?<var> at the start of a pattern binds group 0 as var : string. This may not be the full string if the pattern is unanchored.

A variable is allowed for the universal case and is bound to the matched string. A regular alias is currently not allowed for patterns, since it is not obvious whether is should bind the full string or group 0.

Example

The following prints out times and hosts for SMTP connections to the Postfix daemon:

(* Link with re, re.pcre, lwt, lwt.unix.
   Preprocess with ppx_regexp.
   Adjust to your OS. *)

open Lwt.Infix

let check_line =
  (function%pcre
   | {|(?<t>.*:\d\d) .* postfix/smtpd\[[0-9]+\]: connect from (?<host>[a-z0-9.-]+)|} ->
      Lwt_io.printlf "%s %s" t host
   | _ ->
      Lwt.return_unit)

let () = Lwt_main.run begin
  Lwt_io.printl "SMTP connections from:" >>= fun () ->
  Lwt_stream.iter_s check_line (Lwt_io.lines_of_file "/var/log/syslog")
end

Limitations

No Pattern Guards

Pattern guards are not supported. This is due to the fact that all match cases are combined into a single regular expression, so if one of the patterns succeed, the match is committed before we can check the guard condition.

No Exhaustiveness Check

The syntax extension will always warn if no catch-all case is provided. No exhaustiveness check is attempted. Doing it right would require reimplementing full regular expression parsing and an algorithm which would ideally produce a counter-example.

Bug Reports

The processor is currently new and not well tested. Please break it and file bug reports in the GitHub issue tracker. Any exception raised by generated code except for Match_failure is a bug.