Hum, not easy. You need a parser. This one should do the trick if you're starting from text. 
use HTML::Entities qw( encode_entities );

sub text_to_html {
    my $terms = $_[1];

    my ($terms_pat) =
        map "(?:$_)",
        join '|',
        map quotemeta,
        @$terms;

    ( my $html = $_[0] ) =~
        s{
            ( (?: (?!$terms_pat). )* )
            ( $terms_pat+ )
        |
            ( .+ )
        }{
            defined($1)
                ? encode_entities("$1") .
                  "<b>" . encode_entities("$2") . "</b>"
                : encode_entities("$3")
        }xseg;

    return $html;
}

print text_to_html("best car wash & repair!!", [qw( car repair )]);

[download]

Starting from HTML is much trickier.

Update: Simplified code.
Update: Fixed s// </b> / problem mentioned in a reply.

In reply to Re: Text highlighting (not language syntax) by ikegami
in thread Text highlighting (not language syntax) by vit

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