Help checking for character NOT repeated

I'm trying to create a simple validation but struggling to
see how to
do duplicate checking.

A field can contain 2 or 3 characters from the set C, E, X
The letters can appear in any order
There can be no duplication so CXX or ECE are not valid

I know there only 12 possible options in this particular
example, but
there may be more characters allowed in future.

Is theere a nice neat way to use regexps to validate? I was
looking at
back references but discovered you can't use back refs in a
character
class and certainly can't negate them.


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^(([CEX])(?!2)){2,3}$

The most recent capture against the character class will be
stored in
$2; this capture is followed by the assertion, "Make
sure that the
next character isn't the same as $2."


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On Feb 16, 9:32 am, "SlideGuitar" <d...anthonynassar.com> wrote:
> ^(([CEX])(?!2)){2,3}$
>
> The most recent capture against the character class
will be stored in
> $2; this capture is followed by the assertion,
"Make sure that the
> next character isn't the same as $2."

Hi, Tony:

This pattern may miss out something like:  DED, so I guess,
you need
to add one more option:

  ^(([CEX])(?!2|.2)){2,3}$

Regards,
Xicheng 


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On Feb 16, 6:39 am, "Neff" <fgpsm...gmail.com> wrote:
> I'm trying to create a simple validation but struggling
to see how to
> do duplicate checking.
>
> A field can contain 2 or 3 characters from the set C,
E, X
> The letters can appear in any order
> There can be no duplication so CXX or ECE are not
valid
>
> I know there only 12 possible options in this
particular example, but
> there may be more characters allowed in future.
>
> Is theere a nice neat way to use regexps to validate? I
was looking at
> back references but discovered you can't use back refs
in a character
> class and certainly can't negate them.

One of my reply to your post seems missing, so I will
re-post another
way for your problem.

In fact, it's much easier to check duplication, for
example:

   ([CEX])(?=.*1)

returns TRUE if there is any duplication of chars in [CEX]

Now you want:
1) no any dulicated char in [CEX]
2) exact 2 or 3 chars
3) no any chars in [^CEX]

Let's look at it the other way, negating the constraints
1) any duplicate char in [CEX] exists   ([CEX])(?=.*1)
2) or any char in [^CEX] exists         ^(?=.*[^CEX])
3) has 0 or 1 char in [CEX]             ^[CEX]?$
4) has more than 3 chars                ^[CEX]{3,}$

(4) can be skipped coz if it's TRUE, either (1) or (2) must
be TRUE.

so we can group the above pattern into one, like:

  ^[CEX]?$|^(?=.*[^CEX])|([CEX])(?=.*1)

This is the negation of your needs. use a negation
operator(i.e. 'not'
in Perl) on the above pattern, i.e. a Perl test code under
Linux box
can be like:

echo 'KCXE
XEE
XEC
XXEF
C
EXCC
CE' | perl -lne '
    print "matched: $_" if not m{
       ^[CEX]?$|^(?=.*[^CEX])|([CEX])(?=.*1)
    }x'
matched: XEC
matched: CE

It's not hard to extend this into more than 3 char-class. I
may missed
out something in the above logics, please check them out by
yourself.

Regards,
Xicheng


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On Feb 16, 5:18 pm, "Xicheng Jia" <xich...gmail.com> wrote:
> On Feb 16, 9:32 am, "SlideGuitar"
<d...anthonynassar.com> wrote:
>
> > ^(([CEX])(?!2)){2,3}$
>
> > The most recent capture against the character
class will be stored in
> > $2; this capture is followed by the assertion,
"Make sure that the
> > next character isn't the same as $2."
>
> Hi, Tony:
>
> This pattern may miss out something like:  DED, so I
guess, you need

sorry for the typo, should be:

    strings like 'XEX' instead of 'DED' may be validated

Regards,
Xicheng


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Good catch, Xicheng; I didn't read the original posting
carefully
enough, and didn't see that *no* duplication is allowed.


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Thanks for all your help folks - I think I've got it all
working now.


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