Question

I just started on learning perl, and in the sample code given by my book there is this line:

#!/usr/bin/perl
@lines = `perldoc -u -f atan2`;
foreach (@lines) {
s/\w<([^>]+)>/\U$1/g;
print;
}

the code works, that’s not the problem. Also by observing the input and output I know that this line:

s/\w<([^>]+)>/\U$1/g;

does this

ATAN2 ARCTANGENT TAN TANGENT <--- X<atan2> X<arctangent> X<tan> X<tangent>

and this

MATH::TRIG::TAN <------- C<Math::Trig::tan>

My question is: where does the X and the C come from?

---

As an FYI: here is my output if I don’t use the code:

perldoc -u -f atan2
=over 8

=item atan2 Y,X
X<atan2> X<arctangent> X<tan> X<tangent>

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the C<Math::Trig::tan>
function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

The return value for C<atan2(0,0)> is implementation-defined; consult
your atan2(3) manpage for more information.

=back

And here is the complete output from the code:

[/cygdrive/c/Users/Documents/learn_perl]$ ./hello_world.pl 
=over 8

=item atan2 Y,X
ATAN2 ARCTANGENT TAN TANGENT

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the MATH::TRIG::TAN
function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

The return value for ATAN2(0,0) is implementation-defined; consult
your atan2(3) manpage for more information.

=back

Answer 1

The substitution regex

s/\w<([^>]+)>/\U$1/g;

Finds an alphanumeric character (X and C in the sample text), followed by <, captures any characters that are not >, followed by a >. Then it puts the captured string back, with the escape sequence \U which turns it into upper case.

My question is: where does the X and the C come from?

They were already in the perldoc text. They were however removed by the substitution regex. Perhaps you got input and output confused.