How to match start and end of string with ^ and $ in Perl?

In Perl regular expressions, the anchors ^ and $ are used to match the start and end of a string, respectively. These anchors do not match any actual characters but assert a position in the target string, allowing you to control where your pattern matches.

Understanding ^ and $ anchors

• ^ matches the start of the string (or the start of a line if /m modifier is used).
• $ matches the end of the string (or the end of a line if /m modifier is used).

Common use cases include:

• Matching an entire string exactly with a pattern.
• Making sure a substring only appears at the very beginning or end.
• Anchoring patterns to lines in multiline strings via the /m modifier.

Example: Matching an entire string

If you want to check if a string equals exactly hello, you combine ^ and $ like this:

#!/usr/bin/perl
use strict;
use warnings;

my $string = "hello";

if ($string =~ /^hello$/) {
    print "Matched exactly 'hello'\n";
} else {
    print "No match\n";
}

# Testing with a string that contains 'hello' but not exactly equal
my $string2 = "hello world";

if ($string2 =~ /^hello$/) {
    print "Matched exactly 'hello'\n";
} else {
    print "No match for string2\n";
}

Output:

Matched exactly 'hello'
No match for string2

Key Points About ^ and $

• The ^ matches the position before the first character of the string (or line in /m mode).
• The $ matches the position after the last character of the string (or line in /m mode). It also matches before a newline at the end unless you use the /s modifier.
• Without the /m modifier, ^ and $ always match the start and end of the entire string.
• With the /m modifier, ^ and $ match the start and end of any line (line is a substring separated by newlines).
• To match the very end of string and ignore trailing newline, consider using \z anchor instead of $.

Example with multiline string and /m modifier

my $multiline = "first line\nsecond line\nthird line";

while ($multiline =~ /^(.*)$/mg) {
    print "Line matched: '$1'\n";
}

This code will match each line individually because /m tells ^ and $ to consider newlines as line boundaries.

Common Pitfalls

• For matching exact strings, always use both ^ and $. Omitting them allows the pattern to match anywhere in the string.
• Remember that $ can match just before a trailing newline, so your match may succeed even if extra newline is present at the end.
• If you want to strictly match end of string ignoring trailing newlines, use \z instead of $ (available since Perl 5).
• Without /m, ^ and $ only match the start and end of the whole string, not internal lines.

Mastering ^ and $ anchors is fundamental for precise regex matching in Perl, especially when you want to assert position rather than matching characters directly.