CODEWITHSUNDEEP
phpregex
brewpixels
brewpixels

Reputation: 311

Prevent a number from being the first character in a string with preg_match()

This code works to allow only alphanumeric characters but I want to prevent $name from starting with a number. How do I do this?

$name = "007_jamesbond";

if(preg_match('/[^a-z_\-0-9]/i', $name)){
 echo "invalid name";
}

Upvotes: 0

Views: 1277

Answers (4)

Don't Panic
Don't Panic

Reputation: 41810

It may be clearer to define a pattern for what is valid, and check for things that do not match it.

if(!preg_match('/^[a-z][a-z_\-0-9]*/i', $name)){
   echo "invalid name";
}

//               ^                    anchor to beginning of string
//                 [a-z]              a letter (add underscore here if it's ok too)
//                      [a-z_\-0-9]*  any number of alphanumeric+underscore characters

Upvotes: 1

Exploit
Exploit

Reputation: 6386

Try this: (without using regex)

$first = substr($name, 0,1);
if(is_numeric($first))
{
    echo 'the first character cannot be numeric';
}
else
{
    if(preg_match('/[^a-z_\-0-9]/i', $name))
    {
        echo 'invalid name';
    }
}

Upvotes: -1

Tordek
Tordek

Reputation: 10872

$name = "007_jamesbond";

if(preg_match('/^[^a-z]/i', $name)){
    echo "invalid name";
}

The ^ at the start of a regex means "The start of the string". This regex can be read as: "If $name starts (^) with a character that is not a-z ([^a-z]), it is invalid."

If you want a single regex to match both requirements ("only alphanum, doesn't start with non-letter"), you can use this:

/(^[^a-z]|[^\w\-])/

Upvotes: 0

chris85
chris85

Reputation: 23892

This should do it. Also \w is alphanumeric characters and underscores.

$name = "007\_jamesbond";
if(preg_match('/(^\d|[^\-\w])/', $name)){
    echo "invalid name";
}

Output:

invalid name

Regex101 Demo: https://regex101.com/r/dF0zQ1/1

Update

Should account for decimals and negative numbers as well...

$name = "007\_jamesbond";
if(preg_match('/(^[.\-]?\d|[^\-\w])/', $name)){
    echo "invalid name";
}

Demo: https://regex101.com/r/dF0zQ1/2

Upvotes: 2

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