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# true if $string consists solely of allowed chars (and isn't empty)
$string =~ m/^[\s\d-\(\)\.]+$/;

# or, we could be true if $string contains anything except allowed cha
+rs
$string =~ m/[^\s\d-\(\)\.]/;

$string =~ m/^[\s\d-\(\)\.x]+|(?:[Ee]xt[\.]*)*$/;

sub is_valid {
    # check to see if an entry consists of valid "phone number" chars.
...
    # Ext or ext or x and digits.
    return ( $phone =~ m/^\d+(?:(?:[Ee]xt)|x)*\d$/ );
}

unless ( is_valid($FORM{'phone'} ) {
    send_error("$FORM{'phone'} doesn't meet validity test");
    exit;
}

$phone =~ m{^\d+[\d\-\.\s\(\)]+(?:(?:[Ee]xt[\.]*)|x)*\d$};

# valid number
unless ($FORM{'phone'} =~ /^\d+[\s\d-\(\)\.]+$/) {
...
   # don't die if zero-length!
   die "Bad ext" if length($FORM{'ext'});
}