Learning Perl, Second Edition

15.1. Finding a Substring

Finding a substring depends on where you have lost it. If you happen to have lost it within a bigger string, you're in luck, because index can help you out. Here's how it looks:

$x = index($string,$substring);

Perl locates the first occurrence of substring within string, returning an integer location of the first character. The index value returned is zero-based; if the substring is found at the beginning of the string, you get a 0. If it's one character later, you get a 1, and so on. If the substring can't be found in string, you get -1.

Take a look at these:

$where   = index("hello","e");                 # $where gets 1
$person  = "barney";
$where   = index("fred barney",$person);       # $where gets 5
@rockers = ("fred","barney");
$where   = index(join(" ",@rockers),$person);  # same thing

Notice that both the string being searched and the string being searched for can be a literal string, a scalar variable containing a string, or even an expression that has a string value. Here are some more examples:

$which = index("a very long string","long"); # $which gets 7
$which = index("a very long string","lame"); # $which gets -1

If the string contains the substring at more than one location, the index function returns the leftmost location. To find later locations, you can give index a third parameter. This parameter is the minimum value that will be returned by index, allowing you to look for the next occurrence of the substring that follows a selected position. It looks like this: ...