How can I append instead of replace text during a search-and-replace with Perl?

Question

I am trying to do search-and-replace using a regex in Perl.

The text I am searching for is:

<space>Number<space>NumberNumberNumber

and I want to replace it with:

<space>Number<space>NumberNumberNumberI

I have the following regex which works in finding the string:

\s[0-9]\s[0-9[0-9][0-9]

But what do I do about replacing the string? Basically I just want to append an 'I' to the end.

I'm using:

perl -pi -e "s/\s[0-9]\s[0-9][0-9][0-9]/I/;" testFile

but this is replacing the whole thing with I rather than appending to it.

Answer 1

This is what backreferences are for. Just surround the section of text you want to capture with parentheses. The first set of parentheses are available in $1, the second in $2, and so on.

s/(\s[0-9]\s[0-9]{3})/$1I/

With Perl 5.10 we gained named captures, so you can say

s/(?<bodytext>\s[0-9]\s[0-9]{3})/$+{bodytext}I/

The stuff inbetween < and > is the name. Names become keys in the %+ variable and the values are the captured text.

Another solution is to use a zero-width positive look-behinds

s/(?<=\s[0-9]\s[0-9]{3})/I/

or its, new to Perl 5.10, shorthand \K

s/\s[0-9]\s[0-9]{3}\K/I/

---

Try

perl -pi -e 's/(\s[0-9]\s[0-9][0-9][0-9])/$1I/' filename

If you use double quotes the $1 is interpolated by the shell before Perl ever sees it. If you have problems with something you think should work, it may be a good idea to take a look at what Perl is seeing. You can do this with B::Deparse:

perl -MO=Deparse -pi -e "s/(\s[0-9]\s[0-9][0-9][0-9])/$1I/" filename

That will produce the following output.

BEGIN { $^I = ""; }
LINE: while (defined($_ = <ARGV>)) {
    s/(\s[0-9]\s[0-9][0-9][0-9])/I/;
}
continue {
    print $_;
}
-e syntax OK

From this we can see that $1 is missing. Lets try again with single quotes:

perl -MO=Deparse -pi -e 's/(\s[0-9]\s[0-9][0-9][0-9])/$1I/' filename
BEGIN { $^I = ""; }
LINE: while (defined($_ = <ARGV>)) {
    s/(\s[0-9]\s[0-9][0-9][0-9])/$1I/;
}
continue {
    print $_;
}
-e syntax OK

And once with escaping:

perl -MO=Deparse -pi -e "s/(\s[0-9]\s[0-9][0-9][0-9])/\$1I/" filename
BEGIN { $^I = ""; }
LINE: while (defined($_ = <ARGV>)) {
    s/(\s[0-9]\s[0-9][0-9][0-9])/$1I/;
}
continue {
    print $_;
}
-e syntax OK