how to extract a certain digit from a String using regular expression in php?

Question

I have a String (filename): s_113_2.3gp

How can I extract the number that appears after the second underscore? In this case it's ' 2 ' but in some cases that can be a few digits number.

Also the number of digits that appears after the first underscore can vary so the length of this String is not constant.

Answer 1

You can use a capturing group:

preg_match('/_(\d+)\.\w+$/', $str, $matches);
$number = $matches[1];

\\d+ represents 1 or more digits. The parentheses around that capture it, so you can later retrieve it with $matches[1] . The . needs to be escaped, because otherwise it would match any character but line breaks. \\w+ matches 1 or more word characters (digits, letters, underscores). And finally the $ represents the end of the string and "anchors" the regular expression (otherwise you would get problems with strings containing multiple . ).

This also allows for arbitrary file extensions.

As Ωmega pointed out below there is another possibility, that does not use a capturing group. With the concept of lookarounds, you can avoid matching _ at the start and the \\.\\w+$ at the end:

preg_match('/(?<=_)\d+(?=\.\w+$)/', $str, $matches);
$number = $matches[0];

However, I would recommend profiling, before applying this rather small optimization. But it is something to keep in mind (or rather, to read up on!).

Answer 2

Using regex lookaround it is very short code:

$n = preg_match('/(?<=_)\d+(?=\.)/', $str, $m) ? $m[0] : "";

...which reads: find one or more digits \\d+ that are between underscore (?<=_) and period (?=\\.)