How can I convert a string into a regular expression that matches itself in Perl?

Question

How can I convert a string to a regular expression that matches itself in Perl?

I have a set of strings like these:

Enter your selection:
Enter Code (Navigate, Abandon, Copy, Exit, ?):

and I want to convert them to regular expressions sop I can match something else against them. In most cases the string is the same as the regular expression, but not in the second example above because the ( and ? have meaning in regular expressions. So that second string needs to be become an expression like:

Enter Code \(Navigate, Abandon, Copy, Exit, \?\):

I don't need the matching to be too strict, so something like this would be fine:

Enter Code .Navigate, Abandon, Copy, Exit, ..:

My current thinking is that I could use something like:

s/[\?\(\)]/./g;

but I don't really know what characters will be in the list of strings and if I miss a special char then I might never notice the program is not behaving as expected. And I feel that there should exist a general solution.

Thanks.

Answer 1

As Brad Gilbert commented use quotemeta :

my $regex = qr/^\Q$string\E$/;

or

my $quoted = quotemeta $string;
my $regex2 = qr/^$quoted$/;

Answer 2

There is a function for that quotemeta .

quotemeta EXPR
Returns the value of EXPR with all non-"word" characters backslashed. (That is, all characters not matching /[A-Za-z_0-9]/ will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the \\Q escape in double-quoted strings.

If EXPR is omitted, uses $_ .

Answer 3

From http://www.regular-expressions.info/characters.html :

there are 11 characters with special meanings: the opening square bracket [ , the backslash \\ , the caret ^ , the dollar sign $ , the period or dot . , the vertical bar or pipe symbol | , the question mark ? , the asterisk or star * , the plus sign + , the opening round bracket ( and the closing round bracket )

In Perl (and PHP) there is a special function quotemeta that will escape all these for you.

Answer 4

To put Brad Gilbert 's suggestion into an answer instead of a comment, you can use quotemeta function. All credit to him

Answer 5

Why use a regular expression at all? Since you aren't doing any capturing and it seems you will not be going to allow for any variations, why not simply use the index builtin?

$s1 = 'hello, (world)?!';
$s2 = 'he said "hello, (world)?!" and nothing else.';

if ( -1 != index  $s2, $s1 ) {
    print "we've got a match\n";
}
else {
    print "sorry, no match.\n";
}