Programming Perl, 4th Edition

Beginnings: The \A and ^ Assertions

The \A assertion matches only at the beginning of the string, no matter what. In contrast, though, the ^ assertion always matches at the beginning of string; it can also match with the more traditional meaning of beginning of line: if the pattern uses the /m modifier and the string has embedded newlines, ^ also matches anywhere inside the string immediately following a newline character:

/\Abar/      # Matches "bar" and "barstool"
/^bar/       # Matches "bar" and "barstool"
/^bar/m      # Matches "bar" and "barstool" and "sand\nbar"

Used in conjunction with /g, the /m modifier lets ^ match many times in the same string:

s/^\s+//gm;             # Trim leading whitespace on each line
$total++ while /^./mg;  # Count nonblank lines