FAQ 6.16 How do I efficiently match many regular expressions at once?

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6.16: How do I efficiently match many regular expressions at once?

    ( contributed by brian d foy )

    Avoid asking Perl to compile a regular expression every time you want to
    match it. In this example, perl must recompile the regular expression
    for every iteration of the foreach() loop since it has no way to know
    what $pattern will be.

        @patterns = qw( foo bar baz );
        LINE: while( <> )
                    foreach $pattern ( @patterns )
                    print if /\b$pattern\b/i;
                    next LINE;

    The qr// operator showed up in perl 5.005. It compiles a regular
    expression, but doesn't apply it. When you use the pre-compiled version
    of the regex, perl does less work. In this example, I inserted a map()
    to turn each pattern into its pre-compiled form. The rest of the script
    is the same, but faster.

        @patterns = map { qr/\b$_\b/i } qw( foo bar baz );

        LINE: while( <> )
                    foreach $pattern ( @patterns )
                    print if /\b$pattern\b/i;
                    next LINE;
    In some cases, you may be able to make several patterns into a single
    regular expression. Beware of situations that require backtracking

            $regex = join '|', qw( foo bar baz );

        LINE: while( <> )
                    print if /\b(?:$regex)\b/i;

    For more details on regular expression efficiency, see Mastering Regular
    Expressions by Jeffrey Freidl. He explains how regular expressions
    engine work and why some patterns are surprisingly inefficient. Once you
    understand how perl applies regular expressions, you can tune them for
    individual situations.


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