Str.string_match incorrect

Str.string_match incorrect

[skaller]

This looks like a fairly fundamental bug in Str module..
(so probably I'm missing something ..) 

This program:

let m = Str.regexp "a";;
let b = Str.string_match m "aa" 0;;

print_endline (if b then "YES" else "NO");;

prints YES. Clearly the string "aa" is NOT matched by
the regexp "a".

This *would be* the correct result for the

Str.string_partial_match

function which matches prefixes. However Str.string_match
should match the whole string. [In bugtracker]

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
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Re: [Caml-list] Str.string_match incorrect

[Evan Martin]

On Wed, Dec 22, 2004 at 02:49:30PM +1100, skaller wrote:
> This looks like a fairly fundamental bug in Str module..
> (so probably I'm missing something ..) 
> 
> This program:
> 
> let m = Str.regexp "a";;
> Str.string_match m "aa" 0;;
[evaluates to true]

This is consistent with the docs, which say:
  [string_match r s start] tests whether the characters in s starting at
  position start match the regular expression r.
and in general with how regular expression systems work.  string_match
corresponds to running your automaton directly and seeing whether you
end up in an accept state, while string_partial_match effectively adds
an extra ".*" to the beginning.
(It's more debatable whether this makes sense.)

To force a match across the entire string, use $:
# Str.string_match (Str.regexp "a$") "aa" 0;;
- : bool = false

-- 
Evan Martin
martine@danga.com
http://neugierig.org

Re: [Caml-list] Str.string_match incorrect

[William Lovas]

On Tue, Dec 21, 2004 at 11:44:55PM -0800, Evan Martin wrote:
> This is consistent with the docs, which say:
>   [string_match r s start] tests whether the characters in s starting at
>   position start match the regular expression r.
> and in general with how regular expression systems work.  string_match
> corresponds to running your automaton directly and seeing whether you
> end up in an accept state, while string_partial_match effectively adds
> an extra ".*" to the beginning.
> (It's more debatable whether this makes sense.)

I concur with your assessment, but i think you're characterization of the
semantics of string_partial_match is inaccurate:

    # Str.string_match (Str.regexp "ab") "a" 0;;
    - : bool = false
    # Str.string_partial_match (Str.regexp "ab") "a" 0;;
    - : bool = true
    # Str.string_match (Str.regexp ".*ab") "a" 0;;
    - : bool = false

... unless of course, i've misunderstood you.  I don't think there's a
simple transformation on regular expressions that allow you to emulate
string_partial_match's behavior using only string_match.  (Does anyone
care to prove me wrong? :)

cheers,
William

Re: [Caml-list] Str.string_match incorrect

[Evan Martin]

On Wed, Dec 22, 2004 at 03:00:10AM -0500, William Lovas wrote:
> I concur with your assessment, but i think you're characterization of the
> semantics of string_partial_match is inaccurate:
> 
>     # Str.string_match (Str.regexp "ab") "a" 0;;
>     - : bool = false
>     # Str.string_partial_match (Str.regexp "ab") "a" 0;;
>     - : bool = true
>     # Str.string_match (Str.regexp ".*ab") "a" 0;;
>     - : bool = false
> 
> ... unless of course, i've misunderstood you.  I don't think there's a
> simple transformation on regular expressions that allow you to emulate
> string_partial_match's behavior using only string_match.  (Does anyone
> care to prove me wrong? :)

No misunderstanding except for mine.  You are correct.
(I had assumed, without reading the docs, that the match/partial_match
distinction would be like Python's match/search.)

-- 
Evan Martin
martine@danga.com
http://neugierig.org

Re: [Caml-list] Str.string_match incorrect

[Gerd Stolpmann]

On Mit, 2004-12-22 at 09:00, William Lovas wrote:
> On Tue, Dec 21, 2004 at 11:44:55PM -0800, Evan Martin wrote:
> > This is consistent with the docs, which say:
> >   [string_match r s start] tests whether the characters in s starting at
> >   position start match the regular expression r.
> > and in general with how regular expression systems work.  string_match
> > corresponds to running your automaton directly and seeing whether you
> > end up in an accept state, while string_partial_match effectively adds
> > an extra ".*" to the beginning.
> > (It's more debatable whether this makes sense.)
> 
> I concur with your assessment, but i think you're characterization of the
> semantics of string_partial_match is inaccurate:
> 
>     # Str.string_match (Str.regexp "ab") "a" 0;;
>     - : bool = false
>     # Str.string_partial_match (Str.regexp "ab") "a" 0;;
>     - : bool = true
>     # Str.string_match (Str.regexp ".*ab") "a" 0;;
>     - : bool = false
> 
> ... unless of course, i've misunderstood you.  I don't think there's a
> simple transformation on regular expressions that allow you to emulate
> string_partial_match's behavior using only string_match.  (Does anyone
> care to prove me wrong? :)

The difference between string_match and string_partial_match is quite
simple:

- string_match: whether a prefix of the string matches the regexp

- string_partial_match: whether the string is a prefix of another string
  matching the regexp

On the level of the automaton, string_match is true when the automaton
reaches an accept state whereas string_partial_match is true when the
automaton is in a valid state at the end of the string and there is
still a path to an accept state.

Of course, it is easy to emulate string_partial_match when you already
have the automaton - just make all states accepting that have a path to
an accept state. But there is not a comparably simple rule for the
regular expression. You can do an emulating transformation by generating
the automaton, adding the additional accept states, and transforming the
automaton back to the regexp. There is an algorithm for this, but the
length of the regexp explodes (exponentially when I remember correctly).

Gerd
-- 
------------------------------------------------------------
Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany 
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de
------------------------------------------------------------

Re: [Caml-list] Str.string_match incorrect

[skaller]

On Wed, 2004-12-22 at 19:00, William Lovas wrote:
> On Tue, Dec 21, 2004 at 11:44:55PM -0800, Evan Martin wrote:
> > This is consistent with the docs, which say:
> >   [string_match r s start] tests whether the characters in s starting at
> >   position start match the regular expression r.
> > and in general with how regular expression systems work. 

Then they're simply wrong. The fundamental operation is
to check if a string is in a regular set of strings.
Plainly 'aa' is not in the set { 'a' }.

string_match is actually testing if some prefix of the
argument is in the regular set -- this is core operation
of a lexical analyser.

I'm not against having that operation -- Felix does --
but it leaves us without the most fundamental and
important operation -- validation.

> I concur with your assessment, but i think you're characterization of the
> semantics of string_partial_match is inaccurate:

Looks like partial_match runs thru the whole string,
and if it doesn't encounter an error returns true.

This is the same automaton in which all non-error states
are considered accepting states.

An error state is actually a state where an accepting state 
is not reachable.

There is a transformation: construct the DFA, mark non-error
states accepting, then generate the corresponding regexp.
Whether that is a 'simple' transformation is another issue :))


-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net

Re: [Caml-list] Str.string_match incorrect

[David Brown]

On Thu, Dec 23, 2004 at 02:57:25AM +1100, skaller wrote:
> On Wed, 2004-12-22 at 19:00, William Lovas wrote:
> > On Tue, Dec 21, 2004 at 11:44:55PM -0800, Evan Martin wrote:
> > > This is consistent with the docs, which say:
> > >   [string_match r s start] tests whether the characters in s starting at
> > >   position start match the regular expression r.
> > > and in general with how regular expression systems work. 
> 
> Then they're simply wrong. The fundamental operation is
> to check if a string is in a regular set of strings.
> Plainly 'aa' is not in the set { 'a' }.

This is a strange notion of right and wrong.  The function behaves exactly
as it is specified in the documentation.  It is not difficult to append a
'$' to the regular expression to only match the entire string.

If the function only matched the entire string, yes, appending ".*" would
cause it to match partial matches, but then it would be more difficult to
extract the matched pattern out.

Arguably, another function could be provided to always do full matching of
strings, but I suspect it just isn't used all that much.

Dave

Re: [Caml-list] Str.string_match incorrect

[skaller]

On Thu, 2004-12-23 at 03:58, David Brown wrote:
> On Thu, Dec 23, 2004 at 02:57:25AM +1100, skaller wrote:
> > On Wed, 2004-12-22 at 19:00, William Lovas wrote:
> > > On Tue, Dec 21, 2004 at 11:44:55PM -0800, Evan Martin wrote:
> > > > This is consistent with the docs, which say:
> > > >   [string_match r s start] tests whether the characters in s starting at
> > > >   position start match the regular expression r.
> > > > and in general with how regular expression systems work. 
> > 
> > Then they're simply wrong. The fundamental operation is
> > to check if a string is in a regular set of strings.
> > Plainly 'aa' is not in the set { 'a' }.
> 
> This is a strange notion of right and wrong.  The function behaves exactly
> as it is specified in the documentation.

Huh? I'm really confused. Two people think the documentation
is correct for the behaviour.

I do not -- Saying

	string x is matched by regexp r

means to me the same as

	x is a member of the regular set of strings r denotes

and with that interpretation there is no possible doubt that
the documentation is wrong and should say 

	matches a prefix of the argument string

in order to describe the behaviour.


-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net

Re: [Caml-list] Str.string_match incorrect

[Christopher A. Watford]

> > > Then they're simply wrong. The fundamental operation is
> > > to check if a string is in a regular set of strings.
> > > Plainly 'aa' is not in the set { 'a' }.
> >
> > This is a strange notion of right and wrong.  The function behaves exactly
> > as it is specified in the documentation.
> 
> Huh? I'm really confused. Two people think the documentation
> is correct for the behaviour.
> 
> I do not -- Saying
> 
>        string x is matched by regexp r
> 
> means to me the same as
> 
>        x is a member of the regular set of strings r denotes
> 
> and with that interpretation there is no possible doubt that
> the documentation is wrong and should say
> 
>        matches a prefix of the argument string
> 
> in order to describe the behaviour.
> 

Going by PCRE's documentation the regex /a/ matches: 'a', 'aa',
'baaaaa', 'nota bene', and any other string with 'a' anywhere inside.

>From OCaml's description of string_match:

val string_match: regexp -> string -> int -> bool
[string_match r s start] tests whether the characters in s starting at
position start match the regular expression r. The first character of
a string has position 0, as usual.

This sounds to me like you're doing the PCRE: /a/

if you say:

Str.string_match (Str.regexp "a") "ab" 0 ;;

And in PCRE /a/ matches "ab" or "aa" or "ba".

/^a/ matches "aa", "ab", but not "ba", and /a$/ matches "aa", "ba",
but not "ab". So as far as I can tell, OCaml's string_match is correct
as the manual describes.

-- 
Christopher A. Watford
christopher.watford@gmail.com
http://dorm.tunkeymicket.com

Re: [Caml-list] Str.string_match incorrect

[skaller]

On Sat, 2004-12-25 at 04:40, Christopher A. Watford wrote:

> Going by PCRE's documentation 

no idea why I would do that ...

> Str.string_match (Str.regexp "a") "ab" 0 ;;
> 
> And in PCRE /a/ matches "ab" or "aa" or "ba".

# Str.string_match (Str.regexp "a") "xax" 0;;
- : bool = false

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net

Re: [Caml-list] Str.string_match incorrect

[Christopher A. Watford]

On 25 Dec 2004 11:57:05 +1100, skaller <skaller@users.sourceforge.net> wrote:
> On Sat, 2004-12-25 at 04:40, Christopher A. Watford wrote:
> 
> > Going by PCRE's documentation
> 
> no idea why I would do that ...
> 
> > Str.string_match (Str.regexp "a") "ab" 0 ;;
> >
> > And in PCRE /a/ matches "ab" or "aa" or "ba".
> 
> # Str.string_match (Str.regexp "a") "xax" 0;;
> - : bool = false
> 
> --
> John Skaller, mailto:skaller@users.sf.net
> voice: 061-2-9660-0850,
> snail: PO BOX 401 Glebe NSW 2037 Australia
> Checkout the Felix programming language http://felix.sf.net
> 
> 

>From the documentation, as you have pointed out (now I see what you
mean), it does not make this constraint obvious. It appears that
string_match's documentation should include that an implicit ^ is
added. string_partial_match makes this clear however.

-- 
Christopher A. Watford
christopher.watford@gmail.com
http://dorm.tunkeymicket.com

Re: [Caml-list] Str.string_match incorrect

[skaller]

On Sat, 2004-12-25 at 14:07, Christopher A. Watford wrote:

> >From the documentation, as you have pointed out (now I see what you
> mean), it does not make this constraint obvious. It appears that
> string_match's documentation should include that an implicit ^ is
> added. string_partial_match makes this clear however.

Except even that isn't quite correct -- ^ matches a newline
as well as start of string.

When matching on a 'string' the special handling of newline
is out of place -- newlines are ordinary characters.

When matching on a whole file, as in awk/sed/perl style
of tools, the line orientation makes some sense.

In any case, the bug is a mismatch, IMHO, between what the
function does and what the documentation says it does.

I had expected string_match to match a whole string or fail,
because that's what the documentation says it does.

However there are two compatibility arguments for
changing the documentation instead -- non Ocaml user
expectations (clearly indicated in replies to my post),
and breaking of existing Ocaml code that relies on
the actual behaviour not the documented behaviour,
misreading it as others seem to have done, and assuming
PCRE/Emacs/awk/sed ... or whatever .. semantics.

If that path is chosen I think it would be nice
to provide a function that actually does what the
documentation *originally* specified (that is,
what it says right now): match a whole string.

This is because the obvious hack:

	^r$

does not in fact work if the string contains newlines --
according to the documentation anyhow :)

I'm not a heavy user of Str -- I would never use it
in a serious application. However I used it in
the program I wrote to build test harness for
ExtLib (there is now an extlib-test module in CVS
which tests ExtLib) and it didn't work.

I did indeed fix the problem with ^r$, because I'm
checking filenames which seem unlikely to have any
newlines in them. 

Anyhow .. it's up to Xavier: I posted the original
note into the bugtracker too :)

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net

Re: [Caml-list] Str.string_match incorrect

[William Lovas]

On Sat, Dec 25, 2004 at 11:57:05AM +1100, skaller wrote:
> On Sat, 2004-12-25 at 04:40, Christopher A. Watford wrote:
> 
> > Going by PCRE's documentation 
> 
> no idea why I would do that ...
> 
> > Str.string_match (Str.regexp "a") "ab" 0 ;;
> > 
> > And in PCRE /a/ matches "ab" or "aa" or "ba".
> 
> # Str.string_match (Str.regexp "a") "xax" 0;;
> - : bool = false

*That* i would call a bug.  I think that of

    Str.string_match (Str.regexp "a") "xa" 0;;
    Str.string_match (Str.regexp "a") "ax" 0;;

either both should evaluate to true, or neither should.  Both evaluating to
true would match the typical perl/grep/etc. notion of searching anywhere
inside a string to for a matching substring, whereas neither doing so would
correspond to the mathematical notion of containment in the language of a
regular expression.

William

Re: [Caml-list] Str.string_match incorrect

[Kurt Welgehausen]

> On Tue, Dec 21, 2004 at 11:44:55PM -0800, Evan Martin wrote:
> > This is consistent with the docs, which say:
> >   [string_match r s start] tests whether the characters in s starting at
> >   position start match the regular expression r.
> > and in general with how regular expression systems work. 
>
> Then they're simply wrong. The fundamental operation is
> to check if a string is in a regular set of strings.
> Plainly 'aa' is not in the set { 'a' }.

Str.string_match is almost exactly the same as Python's
re.match:

   # Str.string_match (Str.regexp "a") "ab" 0 ;;
   - : bool = true
   >>> re.match(re.compile("a"), "ab") #succeeds
   <_sre.SRE_Match object at 0x8146f60>

   # Str.string_match (Str.regexp "a") "bab" 0 ;;
   - : bool = false
   >>> re.match(re.compile("a"), "bab") #returns None
   >>>

   # Str.string_match (Str.regexp "a") "bab" 1 ;;
   - : bool = true
   >>> re.match(re.compile("a"), "bab"[1: ])
   <_sre.SRE_Match object at 0x814c748>
   >>> #Python has no parameter to change the start position

Regards

Re: [Caml-list] Str.string_match incorrect

[skaller]

On Thu, 2004-12-23 at 04:26, Kurt Welgehausen wrote:

> Str.string_match is almost exactly the same as Python's
> re.match:

Which is based on PCRE. I checked the Python documentation
and it is *also* incorrect for the same reason.

I do not care what the expected behaviour is, provided
the documentation describes the semantics, and in this
case both Ocaml and Python have it wrong.


-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net