my $v = q/There are 2 boys/; my $regx = '\b(\d+)\b'; my $str = q/$1 of/; # I try to have $1 in a variable and then use the variable in s/// $v =~ s/$regx/$str/g; print $1,"\n"; # print out $1 value print "$v"; The output is as below 3 There are $1 of 10 boys Then I use "$v =~ s/$regx/$str/gee" instead, it output; Bareword found where operator expected at (eval 1) line 1, near "$1 of" (Missing operator before of?) Number found where operator expected at (eval 1) line 1, near "of 10" (Do you need to predeclare of?) 3 There are boys It seems there are no way to let the $1 to be interpolated in this case