Treps Treps - 2 months ago 19x
PHP Question

PHP: preg_match length limit

I use

preg_match('/[a-z]+[0-9]/', strtolower($_POST['value']))
for check that the string contains both letters and numbers. How can I change so it just allow 5 ~ 15 characters?

doesn't work.

Now I tried:

if(preg_match('/^[a-z0-9]{5,15}$/', strtolower($_POST['value']))) {
echo 'Valid!';

else {
echo 'NOOOOO!!!';

If I type "dfgdfgdfg" in the input field it will say "Valid!". The value has to have both letters and numbers, and between 5-15 characters.


The trick is to use a positive lookahead, well in this case you'll need two.

So (?=.*[0-9]) will check if there is at least a digit in your input. What's next? You guessed it, we'll just add (?=.*[a-z]). Now let's merge it with anubhava's answer:


What does this mean in general?

  • (?=.*[0-9]) : check if there is a digit
  • (?=.*[a-z]) : check if there is a letter
  • ^ : match begin of string
  • [a-z0-9]{5,15} : match digits & letters 5 to 15 times
  • $ : match end of string

From the PHP point of view, you should always check if a variable is set:

$input = isset($_POST['value']) ? strtolower($_POST['value']) : ''; // Check if $_POST['value'] is set, otherwise assign empty string to $input

if(preg_match('~(?=.*[0-9])(?=.*[a-z])^[a-z0-9]{5,15}$~', $input)){
    echo 'Valid!';
    echo 'NOOOOO!!!';

Online regex demo