How to replace matched pattern in perl

Question

I'm trying to write a Perl program that accepts an input pattern and print the string with the matched part enclosed with parentheses. For example, if I input the regex ".p" and the string is "developing", it should print "devel(op)ing"

Here is my program:

if (/($pattern)/) {
    $_ =~ s/$1/($1)/;
    print;
} else {
    print "no match\n";
}

When I run it, I got the error message:

Use of uninitialized value $1 in concatenation (.) or string at q1.pl line 12, <> line 1.

The string "devel()ing" was returned. I did some research and seems like the $1 is uninitialized because the string is not matched, but in my case, the $1 should always be initialized since I checked with "if...else..."

FYI, instead of doing a match and then a substitution, you could do omit the match and do the substitution in the conditional. The return value of s/// is the number of substitutions made which in your case will be either 1 (true) or 0 (false). — Grant McLean

choroba choroba · Accepted Answer · 2016-04-04T22:26:18

After the match, $1 contains the string op.

The substitution than matches the op, but there are no parentheses in the search part, so $1 gets cleared, and there's nothing to substitute the op with.

Add the parentheses to create the capture group again:

s/($1)/($1)/;

How to replace matched pattern in perl

2 Answers