Preg Match Issue

[sissy]

Hi folks, how are we all 🙂

I thought the following code would ONLY match A-Za-z0-9 but with the \ and ' character in the $test var it still evalutes true.

What am i missing here :rolleyes:

<?php

$test = "hel\\l'o";

if (preg_match("([[:alnum:]])", $test)) {
     echo 'yeh';
} else {
     echo 'nope';
}

?>

thanks

[cgraz]

it checks if there is an alphanumeric character in your variable, and there is. (all except \ and '). That's why it returns true. Try this

<?php

$test = "-=-=+";

if (preg_match("([[:alnum:]])", $test)) {
     echo 'yeh';
} else {
     echo 'nope';
}

?>

Will print 'nope' becuase there are no alphanumeric characters in that string.

Cgraz

[sissy]

hi cgraz,

ok i understand what ur saying but still doesnt help, i should of explain more i guess.

Basically what am trying to achieva is when adding a username or any similar field to a DB it checks the form input and if the input has anything other then a-zA-Z0-9 its fails and gives an error msg.

<?php

if (preg_match("([[:alnum:]]{6,16})", $_POST['username'])) {
     echo 'good name insert into the DB';
} else {
     echo 'bad name try again';
}

?>

so im a little lost as what to do here 🙂

[xblue]

I think the assertion characters for the beginning and end of the string are what you need. the way you have it, a sequence of 6 alphanumeric characters anywhere in the string meets the requirements, no matter what comes before or after them.

"(^{[[:alnum:]]{6,16}$)"}

[mcgruff]

I wouldn't regex unless I really have to. Try this:

ctype_alpha($_POST['name'])

[xblue]

I think regex are a wonderful powerful thing, and well worth to learn to deal with. however in this fairly simple task ctype_alpha may be a good alternative in combination with checking the length of the string.

[sissy]

Thanks xblue this "(^{[[:alnum:]]{6,16}$)"} worked a treat however i still dont fully comprehend it.

the ^ says u must start with a number or letter and the $ says u must end in a number or letter, by doing this it also eliminates ALL chars except alnum which is perfect and what was wanted, but couldnt understand why "([[:alnum:]]{6,16})" still allowed any char inbetween, thats what confuses me.

mcgruff i thought i would give that ctype_alpha($_POST['name']) ago with strlen as xblue mentioned. After some testing on single execution there around the same speed but on a loop of 100 preg_match is slightly faster and less lines of coding to 🙂 thanks anyway

Anyway it works, thanks again folks.

[mcgruff]

Thanks for the info on the speed test.

I'd always assumed preg_match etc were slower than alternatives (ctype() str_replace() or whatever else might do the same job) so that was interesting to hear about.

[xblue]

Hi Sissy,

the problem is not that your previous pattern allowed some non-alphanumeric character inbetween, but before and after.

some examples what the pattern actually matches

eatmorefruitsandvegetables
+--------------+

eatmorefruits
+-----------+

eat more fruits and vegetables
         +----+

eat more fruit and vegetable
                   +-------+

eat more fruit
--> no match

I hope that sheds some light into it?