A doubt

[mani]

Hi,

I see this code :

while(<STDIN>){

if(/^From: /){

if(/merlyn/){
print "Email from merlyn\n";
}
last;
}
if(/^$/){

last;
}
}

what i cant understand is this :

I know that whatever is read from STDIN is read into $_ I thought if
we are looking for something which has From: we should look for this
pattern in $_
but i am seeing that the $_ never appers in the first if control
statement.
Is it understood that we are refering to $_ everywhere if not
specified in a control statement ???


thanks
\
Mani

 

[Jürgen Exner]

while(<STDIN>){
if(/^From: /){ [...]
what i cant understand is this :

I know that whatever is read from STDIN is read into $_ I thought if
we are looking for something which has From: we should look for this
pattern in $_
but i am seeing that the $_ never appers in the first if control
statement.
Is it understood that we are refering to $_ everywhere if not
specified in a control statement ???

No, that assumption would be wrong. You are refering to $_ if the man page
of the involved function says so.
You just need to understand what functions and operators are involved.

"/^From:/" is the short form of "m(/^From:/)" (sorry, this one you just have
to know).
Now, checking out the documentation of "m" using "perldoc -f m" tells us:

m// The match operator. See the perlop manpage.

Ok, let's check the perlop man page, "perldoc perlop":

m/PATTERN/cgimosx
/PATTERN/cgimosx
Searches a string for a pattern match, and in scalar context
returns true if it succeeds, false if it fails. If no string is
specified via the "=~" or "!~" operator, the $_ string is
searched.

If "/" is the delimiter then the initial "m" is optional.

jue

 

[John W. Krahn]

Hi,

I see this code :

while(<STDIN>){

if(/^From: /){

if(/merlyn/){
print "Email from merlyn\n";
}
last;
}
if(/^$/){

last;
}
}

what i cant understand is this :

I know that whatever is read from STDIN is read into $_ I thought if
we are looking for something which has From: we should look for this
pattern in $_ but i am seeing that the $_ never appers in the first
if control statement.
Is it understood that we are refering to $_ everywhere if not
specified in a control statement ???

Yes, if you add $_ to the program it would look like this:

LOOP: while ( defined( $_ = <STDIN> ) ) {
if ( $_ =~ /^From: / ) {
if ( $_ =~ /merlyn/ ) {
print "Email from merlyn\n";
}
last LOOP;
}
if ( $_ =~ /^$/ ) {
last LOOP;
}
}

The use of $_ is (usually) optional for most operators/functions.


John