Learning Perl, Second Edition

15.2. Extracting and Replacing a Substring

Pulling out a piece of a string can be done with careful application of regular expressions, but if the piece is always at a known character position, this is inefficient. Instead, you should use substr. This function takes three arguments: a string value, a start position (measured like it was measured for index), and a length, like so:

$s = substr($string,$start,$length);

The start position works like index: the first character is zero, the second character is one, and so on. The length is the number of characters to grab at that point: a length of zero means no characters, one means get the first character, two means two characters, and so on. (It stops at the end of the string, so if you ask for too many, it's no problem.) It looks like this:

$hello = "hello, world!";
$grab  = substr($hello, 3, 2);   # $grab gets "lo"
$grab  = substr($hello, 7, 100); # 7 to end, or "world!"

You could even create a "ten to the power of " operator for small integer powers, as in:

$big = substr("10000000000",0,$power+1); # 10 ** $power

If the count of characters is zero, an empty string is returned. If either the starting position or ending position is less than zero, the position is counted that many characters from the end of the string. So -1 for a start position and 1 (or more) for the length gives you the last character. Similarly, -2 for a start position starts with the second-to-last character like this:

$stuff = substr("a very long string",-3,3); ...