Matching Regular Expressions with OCaml Patterns using Mikmatch's syntax

This syntax extension turns

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

into suitable invocations to the ocaml-re library. The patterns are plain strings of the form accepted by Re_pcre, except groups can be bound to variables using the syntax (... as var). The type of var will be string if a match is of the groups is guaranteed given a match of the whole pattern, and string option if the variable is bound to or nested below an optionally matched group.

Additional extensions:

• let%mikmatch name = {|re|} defines reusable patterns for use within other patterns
• type name = {%mikmatch| re |} generates record types with parse/pp functions from patterns
• let destructuring via let%pcre/mikmatch {| regex |} = s, where regex binds values and s is the input string
• /regex/flags for providing compiler flags. i for caseless, u (%mikmatch) unanchored
• Caseless groups via the ~ operator
• Mixed matching (raw strings supported) via function | {%mikmatch| ... |} -> ... | "raw string" -> ...
• Matching guards are supported across all match extensions