help with regular expression

help with regular expression

[zaid khalid]

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Hi Folks

I want some help in writing regular expressions in Ocaml, as I know how to write it in informal way but in Ocaml syntax I can not. For example I want to write "a* | (aba)* ".

Another question if I want the string to be matched against the regular expression to be matched as whole string not as substring what symbol I need to attach to the substring, i.e if I want only concrete strings accepted (like (" ", a , aa , aaa, aba, abaaba), but not ab or not abaa).


Hint I am using (Str.regexp)
Thanks




      

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RE: [Caml-list] help with regular expression

[David Allsopp]

zaid Khalid wrote:
> Hi Folks
>
> I want some help in writing regular expressions in Ocaml, as I know how to write it
> in informal way but in Ocaml syntax I can not. For example I want to write "a* | (aba)* ".

This question would better be posted on the beginners' list - http://caml.inria.fr/resources/forums.en.html#id2267683

Regular Expressions can be done using the Standard Library with the Str module (as you've found) - see http://caml.inria.fr/pub/docs/manual-ocaml/libref/Str.html so your expression above (assuming you have loaded/linked str.cm[x]a) is Str.regexp "a*\\|\\(aba\\)*". The language of regexps is given in the docs for Str.regexp function. Remember to escape backslash characters as the regular expression is given in an OCaml string (so to escape a backslash in your regexp you have to write "\\\\").

> Another question if I want the string to be matched against the regular expression
> to be matched as whole string not as substring what symbol I need to attach to the
> substring, i.e if I want only concrete strings accepted (like (" ", a , aa , aaa, 
> aba, abaaba), but not ab or not abaa).

Use ^ and $ at the beginning and end of your regexp to ensure that it matches the entire string only - "^\\(a*\\|\\(aba\\)*\\)$"

> Hint I am using (Str.regexp)

There are other libraries (e.g. pcre-ocaml) which provide different (I would say more powerful, rather than strictly better!) implementations.


David

Re: help with regular expression

[Sylvain Le Gall]

On 06-12-2010, David Allsopp <dra-news@metastack.com> wrote:
> zaid Khalid wrote:
>>
>
>> Hint I am using (Str.regexp)
>
> There are other libraries (e.g. pcre-ocaml) which provide different (I
> would say more powerful, rather than strictly better!)
> implementations.
>
>

There is also syntax extension like mikmatch, that helps to write regexp
in a very meaningful syntax:

match str with 
| RE bol "a"* | "ab"* eol ->
  true
| _ ->
  false

http://martin.jambon.free.fr/mikmatch-manual.html
http://martin.jambon.free.fr/mikmatch.html

You can use pcre and str with mikmatch.

Regards,
Sylvain Le Gall

Re: help with regular expression

[Dawid Toton]

On 12/06/2010 12:43 PM, zaid khalid wrote:
 > I want some help in writing regular expressions in Ocaml, as I know 
how to write it in informal way but in Ocaml syntax I can not. For 
example I want to write "a* | (aba)* ".
 >
 > Another question if I want the string to be matched against the 
regular expression to be matched as whole string not as substring what 
symbol I need to attach to the substring, i.e if I want only concrete 
strings accepted (like (" ", a , aa , aaa, aba, abaaba), but not ab or 
not abaa).
 >

I also had problems with Str (regexp descriptions being unreadable, 
error-prone and hard to generate dynamically) and decided just to stop 
using Str.
I have a tiny module [1] made with clarity in mind. It is pure OCaml. It 
defines operators like $$ to be used in regexp construction. This way 
syntax of the expressions is checked at compile time. Also, it is 
trivial to build them at run time.
The whole "engine" is contained in a relatively short function 
HRegex.subwords_of_subexpressions, so I believe anybody can hack it 
without much effort.

I haven't measured performance of this implementation. I expect it to be 
slow when processing long strings. It's just OK for my needs so far. 
Anyway, the important part is the module interface. It expresses my 
point of view on this topic.

The code is available in a mercurial repository [2].

The exemple "a* | (aba)* " would become:

open HRegex.Operators

let rx = (!* !$ "a") +$ (!* !$ "aba")

Dawid

[1] 
http://hg.ocamlcore.org/cgi-bin/hgwebdir.cgi/hlibrary/hlibrary/raw-file/tip/HRegex.mli
[2] http://hg.ocamlcore.org/cgi-bin/hgwebdir.cgi/hlibrary/hlibrary

Re: [Caml-list] Re: help with regular expression

[Martin Jambon]

On 12/06/10 05:11, Sylvain Le Gall wrote:
> On 06-12-2010, David Allsopp <dra-news@metastack.com> wrote:
>> zaid Khalid wrote:
>>>
>>
>>> Hint I am using (Str.regexp)
>>
>> There are other libraries (e.g. pcre-ocaml) which provide different (I
>> would say more powerful, rather than strictly better!)
>> implementations.
>>
>>
> 
> There is also syntax extension like mikmatch, that helps to write regexp
> in a very meaningful syntax:
> 
> match str with 
> | RE bol "a"* | "ab"* eol ->
>   true
> | _ ->
>   false

If I understand correctly the original problem, the solution is:

match str with
  | RE ("a"* | "aba"*) eos ->
      (* matches always the beginning of the string,
         eos enforces a match at the end of the string,
         and the vertical bar has the lowest priority
         and so parentheses are needed. *)
      true
  | _ ->
      false


> http://martin.jambon.free.fr/mikmatch-manual.html
> http://martin.jambon.free.fr/mikmatch.html
> 
> You can use pcre and str with mikmatch.

I would recommend the pcre variant mostly for one feature that is not
provided by str:  lazy quantifiers, i.e. "repeat as little as possible
before trying to match what comes next".


Martin

Re: [Caml-list] HELP : with regular expression

[Ashish Agarwal]

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I know you're asking for a solution with Str. Since I can't help with that,
let me give you the Pcre solution instead. Hopefully my explanations will
make up for the lack of examples to help you start using it.

pmatch is the function you care about. Ignore most of the arguments. All you
need to do is pass the regular expression and the string to match against.
The regular expression can be given in either the ~rex or ~pat named
arguments (not both). Use ~pat for hacking, and be sure to compile to ~rex
if you're doing lots of matches. Notice that almost every function has the
same arguments, so once you learn this one the others will make sense too.

extract is the second useful function. pmatch just returns true or false if
the regexp matches or not. extract will give you all the matching
substrings, which is useful to see what your regexp is actually doing (and
of course if you need the matched string for later use).

After you use these two functions you can start looking at the numerous
other ones that let you do more detailed things.

The regular expression I came up with for what you want is "(a+|(aba)+)$".
Let's test it:

$ ocaml
        Objective Caml version 3.12.0

# #require "pcre";;
# open Pcre;;

# pmatch ~pat:"(a+|(aba)+)$" "abaaba";;
- : bool = true

# extract ~pat:"(a+|(aba)+)$" "abaaba";;
- : string array = [|"abaaba"; "abaaba"; "aba"|]

Note that extract gives as its first result the "full match at index 0" and
then all the substrings that match. I'm actually not sure what the use of
this is, and I always set full_match to false to avoid the duplication.

# pmatch ~pat:"(a+|(aba)+)$" "abaa";;
- : bool = true

# extract ~full_match:false ~pat:"(a+|(aba)+)$" "abaa";;
- : string array = [|"aa"; ""|]

# pmatch ~pat:"(a+|(aba)+)$" "abb";;
- : bool = false

# extract ~full_match:false ~pat:"(a+|(aba)+)$" "abb";;
Exception: Not_found.

Hope that helps.


On Mon, Dec 6, 2010 at 6:29 PM, zaid khalid <zaidbenaz@yahoo.com> wrote:

> Hi folks
>
> Thank you for all your replies. I think I am still struggling to find a
> solution to my issue using "Str.regexp", and using Pcre-ocaml needs some
> time to be familiar with as there is no enough examples and discussion on
> it.
>
> Ill put my issue again as if someone can help me to find a solution to it
> with the "Str" .
>
> I want to define regular expression and after that I want to check if
> particular string is a prefix of the given regular expression.
>
> Example: a* | (aba)* so when you test "abaaba" the result will be true
> (complete match) and when we check "abaa" the result is true as well but
> when we check "abb" the result is false.
>
> I look forward to your suggestions.
>
> Cheers,
> Zaid
>
>
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