Question

I have a string that I would like to test to see if it is in the following format.

exactly seven alphabetical OR numerical characters, followed by a '_', followed by 
exactly five alphabetical OR numerical characters, followed by a '_', followed by 
exactly seven alphabetical OR numerical characters, followed by a '_', followed by 
exactly nine alphabetical OR numerical characters. 
No other characters should exist other then the above mentioned characters.

I would like a function that returns true if it matches the description or false if it does not match.

Edit: alpha and numeric characters can be mixed.

Answer 1

The {} will allow you to specify how many times char can be repeated.

The [] lets you list possible characters where - specifies ranges of characters.

The i flag at the end makes it case-insensitive. ^ marks the start of the string and $ marks the end. These prevent anything to be in front of behind of the match.

preg_match('/^[a-z0-9]{7}_[a-z0-9]{5}_[a-z0-9]{7}_[a-z0-9]{9}$/i', $str);