Using a backreference as key in a hashmap within a regex-substitution?-CodePudding

I am learning the Perl language and I stumbled upon the following question: Is it possible to use a backreference as a key in a substitution argument, e.g. something like:

$hm{"Cat"} = "Dog";
while(<>){
s/Cat/$hm{\1}/
print;
}

That is, I want to tell Perl to look up a key which is contained in a capture argument.

I know that this is a silly example. But I am just curious on the question as to whether it is possible to use such a key-lookup with a backreference in a substitution.

CodePudding user response：

Use $1 instead.

While backrefs like \1 work in the substition part of a regex, it only works in string context. The $hm{KEY} is accesses an item in a hash. The KEY part can be a bareword or an expression. In an expression, \1 would be a “reference to a literal scalar with value 1” which would stringify as SCALAR(0x55776153ecb0), not a back-reference as in a string. Instead, we can access the value of captures in the regex with variables like $1.

But that requires us to capture a part of the regex. I would write it as:

s/(Cat)/$hm{$1}/;

As a rule of thumb, only use backrefs like \1 within a regex pattern. Everywhere else use capture variables like $1. If you use warnings, Perl will also tell you that \1 better written as $1, though it wouldn't have detected the particular issue in your case as the \1 was still valid syntax, albeit with different meaning.

CodePudding user response：

If you are looking at really old code, you'll see people using the \1 form on the replacement side of the substitution. Sometimes you'll see it in really new code; it's a Perl 4 thing that still works, but Perl 5 added a warning. If you have warnings turned on, perl will tell you that (although I don't know when this warning started:

$ perl5.36.0 -wpe 's/cat(dog)/\1/'
\1 better written as $1 at -e line 1.

With diagnostics you get even more information about the warning:

$ perl5.36.0 -Mdiagnostics -wpe 's/cat(dog)/\1/'
\1 better written as $1 at -e line 1 (#1)
    (W syntax) Outside of patterns, backreferences live on as variables.
    The use of backslashes is grandfathered on the right-hand side of a
    substitution, but stylistically it's better to use the variable form
    because other Perl programmers will expect it, and it works better if
    there are more than 9 backreferences.

There are many other warnings that Perl uses to show you better ways to do things.

A level above that is perlcritic, which is an opinionated set of policies about what some people find to be good style. It's not a terrible place to start before you develop your own ideas about what works for you or your team.