How can I match at the beginning of any line, including the first, with a Perl regex?

Question

According the the Perl documentation on regexes:

By default, the "^" character is guaranteed to match only the beginning of the string ... Embedded newlines will not be matched by "^" ... You may, however, wish to treat a string as a multi-line buffer, such that the "^" will match after any newline within the string ... you can do this by using the /m modifier on the pattern match operator.

The "after any newline" part means that it will only match at the beginning of the 2nd and subsequent lines. What if I want to match at the beginning of any line (1st, 2nd, etc.)?

EDIT: OK, it seems that the file has BOM information (3 chars) at the beginning and that's what's messing me up. Any way to get ^ to match anyway?

EDIT: So in the end it works (as long as there's no BOM), but now it seems that the Perl documentation is wrong, since it says "after any newline"

Answer 1

The ^ does match the 1st line with the /m flag:

~:1932$ perl -e '$a="12\n23\n34";$a=~s/^/:/gm;print $a'
:12
:23
:34

To match with BOM you need to include it in the match.

~:1939$ perl -e '$a="12\n23\n34";$a=~s/^(\d)/<\1>:/mg;print $a'
12
<2>:3
<3>:4
~:1940$ perl -e '$a="12\n23\n34";$a=~s/^(?:)?(\d)/<\1>:/mg;print $a'
<1>:2
<2>:3
<3>:4

Answer 2

Conceptually, there's assumed to be a newline before the beginning of the string. Consequently, /^a/ will find a letter 'a' at the beginning of a string.

Answer 3

You can use the /^(?:\xEF\xBB\xBF)?/mg regex to match at the beginning of the line anyway, if you want to preserve the BOM.