safer starts_with function

[UnixJunkie]

It should be like this to be less error prone at use:

(* does the string starts with prefix ? *)
let starts_with ~str ~prfx =
Str.string_match (Str.regexp ("^" ^ prfx)) str 0

[UnixJunkie]

I think there is even a possible problem in case ~prfx contains some regexp special characters.
The following might be preferable:

(* does the string starts with prefix ? *)
let starts_with ~str ~prfx =
Str.string_match (Str.regexp_string prfx) str 0

[UnixJunkie]

I am in some other code using this same function, that's why
I notice problems with it (like swapping its parameters inadvertantly or
using mistakenly some special charcters in the ~prfx while you did
not intended them to be compiled in the regexp but matched as-is).

[thelema]

On 11/6/2012 1:52 AM, Francois Berenger wrote:

I think there is even a possible problem in case ~prfx contains some
regexp special characters.
The following might be preferable:

(* does the string starts with prefix ? *)
let starts_with ~str ~prfx =
Str.string_match (Str.regexp_string prfx) str 0

—
Reply to this email directly or view it on GitHub
#97 (comment).

I think this might do unanchored matching; searching for prfx anywhere
in str. A correct solution is to just do a string compare against the
correct length prefix of str:

let starts_width ~str ~prefix = if String.length str < String.length
prefix then false else String.sub str 0 (String.length prefix) = prefix

E.

[UnixJunkie]

No, the matched substring must start at index 0 in the code I gave.
That's correct and I tried it extensively in a toplevel.
I'll send a pull request.

The documentation of the Str module is not that crystal clear.