OCaml regexp "any" matching, where "]" is one of the characters - regex

Question

I'd like to match a string containing any of the characters "a" through "z", or "[" or "]", but nothing else. The regexp should match
"b"
"]abc["
"ab[c"
but not these
"2"
"(abc)"
I tried this:
let content_check(s:string):bool =
Str.string_match (Str.regexp "^[a-z[\]]*$") s 0;;
content_check "]abc[";;
and got warned that the "escape" before the "]" was illegal, although I'm pretty certain that the equivalent in, say, sed or awk would work fine.
Anyhow, I tried un-escaping the cracket, but
let content_check(s:string):bool =
Str.string_match (Str.regexp "^[a-z[]]*$") s 0;;
doesn't work at all, since it should match any of a-z or "[", then the first "]" closes the "any" selection, after which there must be any number of "]"s. So it should match
[abc]]]]
but not
]]]abc[
In practice, that's not what happens at all; I get the following:
# let content_check(s:string):bool =
Str.string_match (Str.regexp "^[a-zA-Z[]]*$") s 0;;
content_check "]abc[";;
content_check "[abc]]]";;
content_check "]abc[";;
val content_check : string -> bool = <fun>
# - : bool = false
# - : bool = false
# - : bool = false
Can anyone explain/suggest an alternative?
#Tim Pietzker's suggestion sounded really good, but appears not to work:
# #load "str.cma" ;;
let content_check(s:string):bool =
Str.string_match (Str.regexp "^[a-z[\\]]*$") s 0;;
content_check "]abc[";;
# val content_check : string -> bool = <fun>
# - : bool = false
#
nor does it work when I double-escape the "[" in the pattern, just in case. :(
Indeed, here's a MWE:
#load "str.cma" ;;
let content_check(s:string):bool =
Str.string_match (Str.regexp "[\\]]") s 0;;
content_check "]";; (* should be true *)

Answer 1

This is not going to really answer your question, but it will solve your problem. With the re library:
let re_set = Re.(rep (* "rep" is the star *) ## alt [
rg 'a' 'z' ; (* the range from a to z *)
set "[]" ; (* the set composed of [ and ] *)
])
(* version that matches the whole text *)
let re = Re.(compile ##
seq [ start ; re_set ; stop ])
let content_check s =
Printf.printf "%s : %b\n" s (Re.execp re s)
let () =
List.iter content_check [
"]abc[" ;
"[abc]]]" ;
"]abc[" ;
"]abc[" ;
"abc##"
]
As you noticed, str from the stdlib is akward, to put it midly. re is a very good alternative, and it comes with various regexp syntax and combinators (which I tend to use, because I think it's easier to use than regexp syntax).

Answer 2

I'm an idiot. (But perhaps the designers of Str weren't so clever either.)
From the "Str" documentation: "To include a ] character in a set, make it the first character in the set."
With this, it's not so clear how to search for "anything except a ]", since you'd have to place the "^" in front of it. Sigh.
:(