Combining 2 preg matches.

[frizzle]

Hi group,

I have a function which validates a string using preg match.
A part looks like

if( !preg_match( '/^([a-z0-9]+(([a-z0-9_-]*)?[a-z0-9])?)$/', $string )
||
preg_match( '/(--|__)+/' ,$string) ) {

i wonder how i could combine those two into one ...
I tried a few different options of putting the second match into the
first one,
using things like [^__]+ etc, but nothing worked for me.
it should prevent double (or more) dashes or underscores behind each
other.
hello-there = ok
hello--there != ok

Any help would be great.

Frizzle.

[Csaba Gabor]

Re: Combining 2 preg matches.

frizzle wrote:

I have a function which validates a string using preg match.
A part looks like
>
if( !preg_match( '/^([a-z0-9]+(([a-z0-9_-]*)?[a-z0-9])?)$/', $string )
||
preg_match( '/(--|__)+/' ,$string) ) {
>
i wonder how i could combine those two into one ...
I tried a few different options of putting the second match into the
first one,
using things like [^__]+ etc, but nothing worked for me.
it should prevent double (or more) dashes or underscores behind each
other.
hello-there = ok
hello--there != ok

Is hello-_there ok?
Is hello_-there ok?
Is _hello-there ok?

If the answer to the above three questions is no, then the following
should do the trick. Note that this implies that the final character
could be a - or _:

if (preg_match('/^([a-z0-9][-_]?)+$/', $string)) { ... }

Csaba Gabor from New York

[Chung Leong]

Re: Combining 2 preg matches.


frizzle wrote:

Hi group,
>
I have a function which validates a string using preg match.
A part looks like
>
if( !preg_match( '/^([a-z0-9]+(([a-z0-9_-]*)?[a-z0-9])?)$/', $string )
||
preg_match( '/(--|__)+/' ,$string) ) {
>
i wonder how i could combine those two into one ...
I tried a few different options of putting the second match into the
first one,
using things like [^__]+ etc, but nothing worked for me.
it should prevent double (or more) dashes or underscores behind each
other.
hello-there = ok
hello--there != ok
>
Any help would be great.
>
Frizzle.

What you need is a lookahead and lookbehind assertion on the dash and
underscore, stating that they're acceptable only if there're letters in
front and behind them:

/^(?:[a-z]|(?<=[a-z])[-_](?=[a-z]))+$/

[frizzle]

Re: Combining 2 preg matches.


Chung Leong wrote:

frizzle wrote:

Hi group,

I have a function which validates a string using preg match.
A part looks like

if( !preg_match( '/^([a-z0-9]+(([a-z0-9_-]*)?[a-z0-9])?)$/', $string )
||
preg_match( '/(--|__)+/' ,$string) ) {

i wonder how i could combine those two into one ...
I tried a few different options of putting the second match into the
first one,
using things like [^__]+ etc, but nothing worked for me.
it should prevent double (or more) dashes or underscores behind each
other.
hello-there = ok
hello--there != ok

Any help would be great.

Frizzle.

>
What you need is a lookahead and lookbehind assertion on the dash and
underscore, stating that they're acceptable only if there're letters in
front and behind them:
>
/^(?:[a-z]|(?<=[a-z])[-_](?=[a-z]))+$/

/^(?:[a-z]|(?<=[a-z])[-_](?=[a-z]))+$/

wowowow, could you explain a little on this ?
like the : and ?<= parts

(i assume 0-9 should still be included??)

Frizzle.

[frizzle]

Re: Combining 2 preg matches.


frizzle wrote:

Chung Leong wrote:

frizzle wrote:

Hi group,
>
I have a function which validates a string using preg match.
A part looks like
>
if( !preg_match( '/^([a-z0-9]+(([a-z0-9_-]*)?[a-z0-9])?)$/', $string )
||
preg_match( '/(--|__)+/' ,$string) ) {
>
i wonder how i could combine those two into one ...
I tried a few different options of putting the second match into the
first one,
using things like [^__]+ etc, but nothing worked for me.
it should prevent double (or more) dashes or underscores behind each
other.
hello-there = ok
hello--there != ok
>
Any help would be great.
>
Frizzle.

What you need is a lookahead and lookbehind assertion on the dash and
underscore, stating that they're acceptable only if there're letters in
front and behind them:

/^(?:[a-z]|(?<=[a-z])[-_](?=[a-z]))+$/

>
/^(?:[a-z]|(?<=[a-z])[-_](?=[a-z]))+$/
>
wowowow, could you explain a little on this ?
like the : and ?<= parts
>
(i assume 0-9 should still be included??)
>
Frizzle.

Still curious after the explanation, but just letting you know it works
axactly as it should ..

Frizzle.

[Rik]

Re: Combining 2 preg matches.

frizzle wrote:

Chung Leong wrote:

>frizzle wrote:

>>Hi group,
>>>
>>I have a function which validates a string using preg match.
>>A part looks like
>>>
>>if( !preg_match( '/^([a-z0-9]+(([a-z0-9_-]*)?[a-z0-9])?)$/',
>>$string )
>>>>>
>>preg_match( '/(--|__)+/' ,$string) ) {
>>>
>>i wonder how i could combine those two into one ...
>>I tried a few different options of putting the second match into the
>>first one,
>>using things like [^__]+ etc, but nothing worked for me.
>>it should prevent double (or more) dashes or underscores behind each
>>other.
>>hello-there = ok
>>hello--there != ok
>>>
>>Any help would be great.
>>>
>>Frizzle.

>>
>What you need is a lookahead and lookbehind assertion on the dash and
>underscore, stating that they're acceptable only if there're letters
>in front and behind them:
>>
> /^(?:[a-z]|(?<=[a-z])[-_](?=[a-z]))+$/

>
>
wowowow, could you explain a little on this ?
like the : and ?<= parts

non-capturing group (usefull when you just want to match, and don't need the
exact matched portion):

http://www.regular-expressions.info/brackets.html

positive lookbehind:

http://www.regular-expressions.info/lookaround.html

$regex ='/ #opening delimiter
^ #start of string
(?: #start of non-capturing group
[a-z] #any character between a and z
| #OR
(?<= #start of positive lookbehind (is preceeded by..)
[a-z] #any character between a and z
) #end of positive lookbehind
[-_] #character - or _ (not incorrect, but probably better
to [_\-],[_-] or [\-_]
(?= #start of positive lookahead
[a-z] #any character between a and z
) #end of positive lookahead
) #end of non-capturing group
+ #1 or more times, greedy
$ #end of string
/x';


Human translation:
The entire(1) string consists of 1 or more (2) characters [a-z] and possibly
the single characters _ or - enclosed by characters in the range [a-z].

(1) by achoring them with ^.....$
(2) by +

(i assume 0-9 should still be included??)

If you want that, yes, just change every [a-z] to [a-z0-9].

Use the /i modifier if you want a match to be case-insensitive.

Grtz,
--
Rik Wasmus

[Rik]

Re: Combining 2 preg matches.

Rik wrote:

$regex ='/ #opening delimiter
^ #start of string
(?: #start of non-capturing group
[a-z] #any character between a and z
| #OR
(?<= #start of positive lookbehind (is preceeded
by..) [a-z] #any character between a and z
) #end of positive lookbehind
[-_] #character - or _ (not incorrect, but probably
better to [_\-],[_-] or [\-_]
(?= #start of positive lookahead
[a-z] #any character between a and z
) #end of positive lookahead
) #end of non-capturing group
+ #1 or more times, greedy
$ #end of string
/x';
>

It just occured to me that, allthough a wonderfull example:

$regex ='/^(?:[a-z]|[a-z][_\-][a-z])+$/';

....will do just fine.

equally so:
$regex ='/^(?:[a-z]+(?:[_\-][a-z]+))+$/';

Lookahead & -behind are unneccessary in this case, and this keep it simple.

Grtz,
--
Rik Wasmus

[frizzle]

Re: Combining 2 preg matches.


Rik wrote:

Rik wrote:

$regex ='/ #opening delimiter
^ #start of string
(?: #start of non-capturing group
[a-z] #any character between a and z
| #OR
(?<= #start of positive lookbehind (is preceeded
by..) [a-z] #any character between a and z
) #end of positive lookbehind
[-_] #character - or _ (not incorrect, but probably
better to [_\-],[_-] or [\-_]
(?= #start of positive lookahead
[a-z] #any character between a and z
) #end of positive lookahead
) #end of non-capturing group
+ #1 or more times, greedy
$ #end of string
/x';

>
>
It just occured to me that, allthough a wonderfull example:
>
$regex ='/^(?:[a-z]|[a-z][_\-][a-z])+$/';
>
...will do just fine.
>
equally so:
$regex ='/^(?:[a-z]+(?:[_\-][a-z]+))+$/';
>
Lookahead & -behind are unneccessary in this case, and this keep it simple.
>
Grtz,
--
Rik Wasmus

Wow, thanks for the explanation!
Nice link there as well. Going right into my bookmarks.

Frizzle.

[Chung Leong]

Re: Combining 2 preg matches.

Rik wrote:

It just occured to me that, allthough a wonderfull example:
>
$regex ='/^(?:[a-z]|[a-z][_\-][a-z])+$/';
>
...will do just fine.
>
equally so:
$regex ='/^(?:[a-z]+(?:[_\-][a-z]+))+$/';
>
Lookahead & -behind are unneccessary in this case, and this keep it simple.

Good point. It doesn't make sense to use assertions when you'll capture
the matches anyway.

[frizzle]

Re: Combining 2 preg matches.


Chung Leong wrote:

Rik wrote:

It just occured to me that, allthough a wonderfull example:

$regex ='/^(?:[a-z]|[a-z][_\-][a-z])+$/';

...will do just fine.

equally so:
$regex ='/^(?:[a-z]+(?:[_\-][a-z]+))+$/';

Lookahead & -behind are unneccessary in this case, and this keep it simple.

>
Good point. It doesn't make sense to use assertions when you'll capture
the matches anyway.

Somehow, i believe Rik's solution, gave me problems ...

'/^(?:[a-z0-9]|[a-z0-9][_\-][a-z0-9])+$/'; gave problems.
'/^(?:[a-z0-9]|(?<=[a-z0-9])[-_](?=[a-z0-9]))+$/' didn't.

An example string that gave problems is:
really_a_made_u p_string

So i used Chung's option.

Frizzle.

[Rik]

Re: Combining 2 preg matches.

frizzle wrote:

>>$regex ='/^(?:[a-z]|[a-z][_\-][a-z])+$/';
>>>
>>...will do just fine.
>>>
>>equally so:
>>$regex ='/^(?:[a-z]+(?:[_\-][a-z]+))+$/';
>>>
>>Lookahead & -behind are unneccessary in this case, and this keep it
>>simple.

>Good point. It doesn't make sense to use assertions when you'll
>capture
>the matches anyway.

Somehow, i believe Rik's solution, gave me problems ...
>
'/^(?:[a-z0-9]|[a-z0-9][_\-][a-z0-9])+$/'; gave problems.
'/^(?:[a-z0-9]|(?<=[a-z0-9])[-_](?=[a-z0-9]))+$/' didn't.
>
An example string that gave problems is:
really_a_made_u p_string

Ah, forgot that in [a-z0-9][_\-][a-z0-9] the character on the right is
already matched, so it won't work as a start for the second _ in _a_....

This one should still be working though:
$regex ='/^(?:[a-z0-9]+(?:[_\-][a-z0-9]+)*)$/';

Grtz,
--
Rik Wasmus

[frizzle]

Re: Combining 2 preg matches.


Rik wrote:

frizzle wrote:

>$regex ='/^(?:[a-z]|[a-z][_\-][a-z])+$/';
>>
>...will do just fine.
>>
>equally so:
>$regex ='/^(?:[a-z]+(?:[_\-][a-z]+))+$/';
>>
>Lookahead & -behind are unneccessary in this case, and this keep it
>simple.
Good point. It doesn't make sense to use assertions when you'll
capture
the matches anyway.

Somehow, i believe Rik's solution, gave me problems ...

'/^(?:[a-z0-9]|[a-z0-9][_\-][a-z0-9])+$/'; gave problems.
'/^(?:[a-z0-9]|(?<=[a-z0-9])[-_](?=[a-z0-9]))+$/' didn't.

An example string that gave problems is:
really_a_made_u p_string

>
>
Ah, forgot that in [a-z0-9][_\-][a-z0-9] the character on the right is
already matched, so it won't work as a start for the second _ in _a_....
>
This one should still be working though:
$regex ='/^(?:[a-z0-9]+(?:[_\-][a-z0-9]+)*)$/';
>
Grtz,
--
Rik Wasmus

ok, dankjewel / thanks a lot.

Frizzle.

[Chung Leong]

Re: Combining 2 preg matches.

Rik wrote:

Ah, forgot that in [a-z0-9][_\-][a-z0-9] the character on the right is
already matched, so it won't work as a start for the second _ in _a_....

You know, I thought that was the problem initially, but then remembered
that the regular expression engine does backtracking in order to
maximise any match. When it encounters the underscore after assigning
the letter to the first subpattern, it's supposed to abandon the
previous match, backtrack to the letter, and go down the second branch.

[Rik]

Re: Combining 2 preg matches.

Chung Leong wrote:

Rik wrote:

>Ah, forgot that in [a-z0-9][_\-][a-z0-9] the character on the right
>is already matched, so it won't work as a start for the second _ in
>_a_....

>
You know, I thought that was the problem initially, but then
remembered that the regular expression engine does backtracking in
order to
maximise any match. When it encounters the underscore after assigning
the letter to the first subpattern, it's supposed to abandon the
previous match, backtrack to the letter, and go down the second
branch.

Yes and no. It does exactly what you say, but it is simply not valid:

With the pattern:
'/^(?:[a-z0-9]|[a-z0-9][_\-][a-z0-9])+$/';
one states the entire string can be build by either [a-z0-9](1)OR
[a-z0-9][_\-][a-z0-9](2), think of them as blocks.

Let's examine it (not entirely how it works, but this instance close
enough):
(fixed width font is handy now:)
positions: 123456789012345 678901234567890
string: really_a_made_u p_string
match1: 111111_error, let's try the other option.
match2: 111112--_error, no other matches possible.

There is no possibility for a match with either (1) or (2) at the second _,
and no other options to match instead at the beginning of the string.

Grtz,
--
Rik Wasmus

[Chung Leong]

Re: Combining 2 preg matches.

Rik wrote:

Yes and no. It does exactly what you say, but it is simply not valid:
>
With the pattern:
'/^(?:[a-z0-9]|[a-z0-9][_\-][a-z0-9])+$/';
one states the entire string can be build by either [a-z0-9](1)OR
[a-z0-9][_\-][a-z0-9](2), think of them as blocks.
>
Let's examine it (not entirely how it works, but this instance close
enough):
(fixed width font is handy now:)
positions: 123456789012345 678901234567890
string: really_a_made_u p_string
match1: 111111_error, let's try the other option.
match2: 111112--_error, no other matches possible.
>
There is no possibility for a match with either (1) or (2) at the second _,
and no other options to match instead at the beginning of the string.

Ah! I missed the single letter case.