Some examples: 
    $_ = "foo bar foo";
    $pat1 = "(foo)";
    $pat2 = "(bar)";
    /$pat1/ && /$pat2/;          # Sets $1 (twice).
    /^(?=.*?$pat1)(?=.*?$pat2)/; # Sets $1 and $2.

    $pat1 = "(foo)";
    $pat2 = "(baz)";
    /$pat1/ && /$pat2/;          # Sets $1.
    /^(?=.*?$pat1)(?=.*?$pat2)/; # Doesn't set $1 or $2.

    $pat1 = "(foo)";
    $pat2 = "( )\\g{1}";
    /$pat1/ && /$pat2/;          # No match.
    /^(?=.*?$pat1)(?=.*?$pat2)/; # Match!

[download]
As for the leading ^ - it only has a speed influence, and only if there's no match. But it can be a significant speed difference if the string is long, so it's better not to omit it.

In reply to Re^6: AND and OR on Regular Expressions by JavaFan
in thread AND and OR on Regular Expressions by vitoco

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