What regex can I use to validate a number between 0 and 255?

Question

I want to validate number in range of 0-255

I have this expression

'/^([0-1]?[0-9]?[0-9])|([2][0-4][0-9])|(25[0-5])$/'

But this accepts any number... And this works:

'/(^[0-1]?[0-9]?[0-9]$)|(^[2][0-4][0-9]$)|(^25[0-5]$)/'

why do I have to have ^ and $ for each option?

edit: I have it, but I cannot answer my question, so - ^ and $ have higher priority than |, so /^(...)$/ helped

Answer 1

Note: @alex's answer is the way to go. Validating a number range with regex is like using a jackhammer to catch a fly.

But nevertheless I wanted to provide an explanation for this "problem".

---

You don't have to, you just have to group it correctly, which means you have to group the alternation:

'/^(([0-1]?[0-9]?[0-9])|([2][0-4][0-9])|(25[0-5]))$/'
// ^                                             ^

Otherwise the expression will be interpreted as follows (logically)

(^number) OR (number) OR (number$)

With the brackets, it will be

^(number OR number OR number)$

Answer 2

Don't use a regex for validating a number range.

Just use a condition...

if ($number >= 0 AND $number <= 255) {
   ...
}

This will ensure the number is between 0 and 255 inclusively, which is what your regex appears to be doing.

To answer your question specifically, it doesn't work because you need to wrap the whole thing with a capturing group otherwise the regex engine will do an OR of each individual regex...

/^([0-1]?[0-9]?[0-9]|[2][0-4][0-9]|25[0-5])$/

Also note that $ will match before any trailing \n. Use \z if you really want to match at the end of the string.