Perl正規表現の罠と罠はずし その2

マッチングパターン「.」に改行文字を含める場合、マッチング修飾子「s」を用いるが、行末アンカー「$」を付けた場合に、キチンと行末（改行の前もしくは文字列の末尾）を認識するマッチングパターンは以下のようになる。

(?:(?:$)|(?=[\n]))

行頭・行末それぞれを正しく認識できるマッチングパターンを使ったプログラム例

$text = "12345\nABCDEFG\n12345\n";
print '=' x 10 . '(1-0)' . '=' x 10 . "\n";
print $text;
$text =~ s/^ABCDE/x/gs;
print '=' x 10 . '(1-1)' . '=' x 10 . "\n";
print $text;
$text =~ s/(?:(?:^)|(?<=[\n]))ABCDE/abcde/gs;
print '=' x 10 . '(1-2)' . '=' x 10 . "\n";
print $text;

$text = "12345\nABCDEFG\n12345\n";
print '=' x 10 . '(2-0)' . '=' x 10 . "\n";
print $text;
$text =~ s/^1234/6789/gs;
print '=' x 10 . '(2-1)' . '=' x 10 . "\n";
print $text;
$text =~ s/(?:(?:^)|(?<=[\n]))1234/6789/gs;
print '=' x 10 . '(2-2)' . '=' x 10 . "\n";
print $text;
$text =~ s/(?:(?:^)|(?<=[\n]))6789/1234/gs;
print '=' x 10 . '(2-3)' . '=' x 10 . "\n";
print $text;

$text = "12345\nABCDEFG\n12345\n";
print '=' x 10 . '(3-0)' . '=' x 10 . "\n";
print $text;
$text =~ s/EFG$/x/gs;
print '=' x 10 . '(3-1)' . '=' x 10 . "\n";
print $text;
$text =~ s/EFG(?:(?:$)|(?=[\n]))/efg/gs;
print '=' x 10 . '(3-2)' . '=' x 10 . "\n";
print $text;

$text = "12345\nABCDEFG\n12345\n";
print '=' x 10 . '(4-0)' . '=' x 10 . "\n";
print $text;
$text =~ s/2345$/7890/gs;
print '=' x 10 . '(4-1)' . '=' x 10 . "\n";
print $text;
$text =~ s/2345(?:(?:$)|(?=[\n]))/7890/gs;
print '=' x 10 . '(4-2)' . '=' x 10 . "\n";
print $text;
$text =~ s/7890(?:(?:$)|(?=[\n]))/2345/gs;
print '=' x 10 . '(4-3)' . '=' x 10 . "\n";
print $text;

実行結果

==========(1-0)==========
12345
ABCDEFG
12345
==========(1-1)==========
12345
ABCDEFG
12345
==========(1-2)==========
12345
abcdeFG
12345
==========(2-0)==========
12345
ABCDEFG
12345
==========(2-1)==========
67895
ABCDEFG
12345
==========(2-2)==========
67895
ABCDEFG
67895
==========(2-3)==========
12345
ABCDEFG
12345
==========(3-0)==========
12345
ABCDEFG
12345
==========(3-1)==========
12345
ABCDEFG
12345
==========(3-2)==========
12345
ABCDefg
12345
==========(4-0)==========
12345
ABCDEFG
12345
==========(4-1)==========
12345
ABCDEFG
17890
==========(4-2)==========
17890
ABCDEFG
17890
==========(4-3)==========
12345
ABCDEFG
12345