php regex: phone number 7-12 digits may contain hy

Question

I tried the solution for phone numbers with 7-12 digits that may contain spaces or hypens in the following link. The first and last character has to be a number.

Regular expression to match 7-12 digits; may contain space or hyphen

However, i'm not understanding the regex well.

$phone_pattern="/^\d(?:[-\s]?\d){6,11}$/";

what does the ":" mean here?

How is this regex able to exclude the hypens and spaces from the 6 to 11 characters?

Help help is highly appreciated

Answer 1

The : is part of the (?: ... ) - which means "non-capturing group" - it groups content but does not create a backreference to it (i.e. $1, $2, etc) like normal grouping does.

In that regex it will match from 6 up to 11 characters, including the heiphens and spaces - it meaning something like 12-------34 would match. I suggest using a more strict pattern:

/^\d{7,12}$/

This will only match the digits. To allow for heiphens and spaces with this match, but only get the number you want, you can use it like this:

<?php
$pattern = '/^\d{7,12}$/';
$string = '123-456 789';
$ignoreCharacters = array(' ', '-');

preg_match($pattern, str_replace($ignoreCharacters, $string);

Answer 2

Have you tried a testing engine like regexpal (there are others available also) I frequently use this to test various strings against expressions to make sure they are behaving as expected.

My understanding is that in this case the : is not acting alone it is acting in conjunction with the ?

The ? in this circumstance does not mean Preceding zero or one times it is a modifier meaning give this group a new meaning which in conjunction with the modifier : turn off capture means you want this to be a non capturing group expression.

The effect that this has is that when placing part of an expression inside () it by default causes capture but ?: switches this off.

Thus (?:[-\s]?\d) becomes a non capturing group expression.

Note captured groups are used with back references and most regex engines support up to 9 back references.

So removing capture speeds up the matching process and allows you to elect not to refer back to that group saving one of your 9 references for a group that you really do want to refer back to.

Answer 3

It's understandable how that can be confusing. The (?: ... ) actually denotes a "non-capturing group," as opposed to the ( ... ), which is a "capturing group". If you're only testing strings against regexes, not capturing substrings, then the two are effectively the same for your purposes.

It doesn't help that there also exist (?= ... ), (?! ... ), (?<= ... ), (?<! ... ), and (?<foo> ... ), which all mean different things, too.

A lot to learn, but rewarding for sure!