Question

I have a string that contains 5 words. In the string one of the words is a Ham Radio Call Sign and can be anyone of the thousands of call signs in the US. In order to extract the Call Sign from the string I need to utilize the below pattern. The Call Sign I need to extract can be in any of the 5 positions in the string. The number is never the first character and the number is never the last character. The string is actually put together from an Array since it is originally read from a text file.

$string = $word[1] $word[2] $word[3] etc.... 

So the search can be either done on the whole string or each piece of the array.

Patterns:
1 Number and 3 Letters Example: AB4C A4BC
1 Number and 4 Letters Example: A4BCD
1 Number and 5 Letters Example: AB4CDE

I have tried everything I can think of and search till I cant search no more. I am sure I am over thinking this.

Answer 1

A two-step regular expression like this would do it:

$str = "hello A4AB there BC5AD";
$signs = array();
preg_match_all('/[A-Z][A-Z\d]{1,3}[A-Z]/', $str, $possible_signs);
foreach($possible_signs[0] as $possible_sign)
    if (preg_match('/^\D+\d\D+$/', $possible_sign))
        array_push($signs, $possible_sign);

print_r($signs); //Array ([0] => A4AB [1] => BC5AD)

Explanation

This is a regular expression approach, using two patterns. I don’t think it could be done with one and still satisfy the exact requirements of the matching rules.

The first pattern enforces the following requirements:

• substring starts and ends with a capital letter
• substring contains only other capital letters or numbers between the first and last letter
• substring is, overall, not more than 6 characters long

What I can’t do in that same pattern, for complex REGEX reasons I won’t go into (unless someone knows a way and can correct me), is enforce that only one number is contained.

@jeroen’s answer does enforce this in a single pattern, but in turn does not enforce the correct length of the substring. Either way, we need a second pattern.

So after grabbing the initial matches, we loop over the results. We then apply each to a second pattern that enforces simply that there is only one number in the substring.

If so, we green-light the substring and it’s added to the $signs array.

Hope this helps.