trying out escape characters

[Griff]

i'm using ActivePerl, version 5.6.1

My question is:

How come

print "\ "

prints a backslash followed by a space

but

print "\z"

prints the z without a backslash ?

As neither \z nor \+space are Perl escape sequences as far as I know,
i would expect them both to be processed identically ?

Thanks,

Griff

[nobull]

gr*****@aol.com (Griff) wrote in message news:<d6**************************@posting.google. com>...

How come

print "\ "

prints a backslash followed by a space
It does not. It prints a space.
print "\z"

prints the z without a backslash ?
Yeah, but it emits a warning.
As neither \z nor \+space are Perl escape sequences as far as I know,
i would expect them both to be processed identically ?

If you have warnings switched off then they appear to be processed
identically.

However, you should not have warnings switched off.

Any non-letter preceded by a backslash represents that literal
character, always.

Any letter preceded by a backslash is potentially a Perl escape
sequence. There may be no \z yet but there may be one in future.
Hense you get a warning. Until and unless \z is defined it is
interpreted as a literal z. If this offends you then you can always
promote the warning to an error.

This newsgroup does not exist (see FAQ). Please do not start threads
here.

[Griff]

(1) apologies for starting a thread in non-existent NG, I will use
comp.lang.perl.misc in future.

(2) i think I know where my confusion arose...

print "\Qz\E";
print "\n";
print "\Q \E";
print "\n";

produces three lines of output,
line 1 : z
line 2 : \
line 3 : (newline)

Why does output line 2 contain a backslash when output line 1 doesn't
?

thanks - Griff

[thundergnat]

Griff wrote:

(1) apologies for starting a thread in non-existent NG, I will use
comp.lang.perl.misc in future.

(2) i think I know where my confusion arose...

print "\Qz\E";
print "\n";
print "\Q \E";
print "\n";

produces three lines of output,
line 1 : z
line 2 : \
line 3 : (newline)

Why does output line 2 contain a backslash when output line 1 doesn't
?

thanks - Griff

Actually line 2 has a has a backslash followed by a space. The \Q..\E
construct all non word characters with a backslash. A space is not a
word character, hence it returns "\ ". To illustrate, try somthing like:

print "\Qab_cd1:2'3/4? 5\E";
This group is defunct. Please use comp.lang.perl.misc in future.

[thundergnat]

Griff wrote:

(1) apologies for starting a thread in non-existent NG, I will use
comp.lang.perl.misc in future.

(2) i think I know where my confusion arose...

print "\Qz\E";
print "\n";
print "\Q \E";
print "\n";

produces three lines of output,
line 1 : z
line 2 : \
line 3 : (newline)

Why does output line 2 contain a backslash when output line 1 doesn't
?

thanks - Griff

Actually line 2 has a has a backslash followed by a space. The \Q..\E
construct prepends a backslash to all non word characters. A space is
not a word character, hence it returns "\ ". To illustrate, try somthing
like:

print "\Qab_cd 1:2'3/4?5\E";
This group is defunct. Please use comp.lang.perl.misc in future.

[Joe Smith]

Griff wrote:

(2) i think I know where my confusion arose...

print "\Qz\E";

That's not their purpose. \Q and \E should only be used
a) in the match operator (m//).
b) in the lefthand part of the substitue operator (s///).
c) in creating regular expressions via qr//, to be
used in one of the two above cases.

-Joe

[nobull]

Joe Smith <Jo*******@inwap.com> wrote in message news:<O%DUc.7190$Fg5.7106@attbi_s53>...

Griff wrote:

(2) i think I know where my confusion arose...

print "\Qz\E";

That's not their purpose. \Q and \E should only be used
a) in the match operator (m//).
b) in the lefthand part of the substitue operator (s///).
c) in creating regular expressions via qr//, to be
used in one of the two above cases.

That's not enirely true. quotemeta() (aka \Q...\E) can also be useful
in constructing strings that are to passed to eval() or system().