13.1. Finding a Substring with index
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 the index function can help you out. Here's how it looks:
$where = index($big, $small);
Perl locates the first occurrence of the small string within the big string, returning an integer location of the first character. The character position returned is a zero-based value. If the substring is found at the beginning of the string, index returns 0. If it's one character later, the return value is 1, and so on. If the substring can't be found at all, the return value is -1.[*] In this example, $where gets 6:
my $stuff = "Howdy world!"; my $where = index($stuff, "wor");
Another way you could think of the position number is the number of characters to skip over before getting to the substring. Since $where is 6, we know we have to skip over the first six characters of $stuff before we find wor.
The index function always reports the location of the first found occurrence of the substring. But you can tell it to start searching at a later point than the start of the string by using the optional third parameter, which tells index to start at that position:
my $stuff = "Howdy world!"; my $where1 = index($stuff, "w"); # $where1 gets 2 my $where2 = index($stuff, "w", $where1 + 1); # $where2 gets 6 my $where3 = index($stuff, "w", $where2 + 1); # $where3 gets -1 (not found)
(You wouldn't normally search repeatedly for a substring without using a loop.) That third parameter is giving a minimum value for the return value; if the substring can't be found at that position or later, the return value will be -1.
Once in a while, you might prefer to have the last found occurrence of the substring.[*] You can get that with the rindex function. In this example, we can find the last slash, which turns out to be at position 4 in a string:
my $last_slash = rindex("/etc/passwd", "/"); # value is 4
The rindex function has an optional third parameter; in this case, it gives the maximum permitted return value:
my $fred = "Yabba dabba doo!"; my $where1 = rindex($fred, "abba"); # $where1 gets 7 my $where2 = rindex($fred, "abba", $where1 - 1); # $where2 gets 1 my $where3 = rindex($fred, "abba", $where2 - 1); # $where3 gets -1