I have a special char in my variable like this
my $var = "\n";
my $var_tab = "\t";
I need to print out not this character but character value/code (\t,\n).
I have tried to add one more slash, but it doesn't work.
my $var.="\\";
How can I print value of special char?
It might help us understand the context if you explained why you want this and what you're trying to achieve.
If you simple want to dump out the value of a variable and have the special characters be visible and readable then perhaps Data::Dumper would help:
use Data::Dumper
$var = "\n";
$var_tab = "\t";
$Data::Dumper::Terse = 1;
$Data::Dumper::Useqq = 1;
print Data::Dumper::qquote($var); # "\n"
print Data::Dumper::qquote($var_tab); # "\t"
If you want print the special character value, Try regex for substitute the \ with empty.Like
my $var = "\\n";
$var =~s/\\//g;
print "$'";
Else If you want print the \n . Just use the single quotes instead of double quotes. or use forward slashes within double quotes.
my $var = "\\n"; #slashes for escape the special character
my $var_tab = '\t'; #single quotes also escape the special character
print "$var $var_tab ";
Related
I have a variable that contains a slash and a t.
my $var = "\\t";
I want to convert that to a tab. How do I do that?
use Data::Dumper;
use Term::ReadLine;
my $rl = Term::ReadLine->new();
my $var = $rl->readline( 'Enter \t:' );
print Dumper $var;
The following is the simplest solution:
$var = "\t" if $var eq "\\t";
If you want to do this no matter where the sequence appears in the string, you could use
$var =~ s/\\t/\t/g;
But it sounds like you're not asking the right question. Nothing supports \t and nothing else. At the very least, I would also expect \\ to produce \. Are you perhaps trying to parse JSON? If so, there are number of other escape sequences you need to worry about.
How can I print a string (single-quoted) containing double-backslash \\ characters as is without making Perl somehow interpolating it to single-slash \? I don't want to alter the string by adding more escape characters also.
my $string1 = 'a\\\b';
print $string1; #prints 'a\b'
my $string1 = 'a\\\\b';
#I know I can alter the string to escape each backslash
#but I want to keep string as is.
print $string1; #prints 'a\\b'
#I can also use single-quoted here document
#but unfortunately this would make my code syntactically look horrible.
my $string1 = <<'EOF';
a\\b
EOF
print $string1; #prints a\\b, with newline that could be removed with chomp
The only quoting construct in Perl that doesn't interpret backslashes at all is the single-quoted here document:
my $string1 = <<'EOF';
a\\\b
EOF
print $string1; # Prints a\\\b, with newline
Because here-docs are line-based, it's unavoidable that you will get a newline at the end of your string, but you can remove it with chomp.
Other techniques are simply to live with it and backslash your strings correctly (for small amounts of data), or to put them in a __DATA__ section or an external file (for large amounts of data).
If you are mildly crazy, and like the idea of using experimental software that mucks about with perl's internals to improve the aesthetics of your code, you can use the Syntax::Keyword::RawQuote module, on CPAN since this morning.
use syntax 'raw_quote';
my $string1 = r'a\\\b';
print $string1; # prints 'a\\\b'
Thanks to #melpomene for the inspiration.
Since the backslash interpolation happens in string literals, perhaps you could declare your literals using some other arbitrary symbol, then substitute them for something else later.
my $string = 'a!!!b';
$string =~ s{!}{\\}g;
print $string; #prints 'a\\\b'
Of course it doesn't have to be !, any symbol that does not conflict with a normal character in the string will do. You said you need to make a number of strings, so you could put the substitution in a function
sub bs {
$_[0] =~ s{!}{\\}gr
}
my $string = 'a!!!b';
print bs($string); #prints 'a\\\b'
P.S.
That function uses the non-destructive substitution modifier /r introduced in v5.14. If you are using an older version, then the function would need to be written like this
sub bs {
$_[0] =~ s{!}{\\}g;
return $_[0];
}
Or if you like something more readable
sub bs {
my $str = shift;
$str =~ s{!}{\\}g;
return $str;
}
I would like to use the value of a variable (fixed by a command line option for instance) as a list separator, enabling that value to be a special character (newline, tabulation, etc.).
Unfortunately the naïve approach does not work due to the fact that the two following print statement behave differentely :
my #tab = ("a","b","c");
# block 1 gives expected result:
# a
# b
# c
{
local $" = "\n"; #" let us please the color syntax engine
print "#tab";
}
# block 2 gives unwanted result:
# a\nb\nc
{
use Getopt::Long;
my $s;
GetOptions('separator=s' => \$s);
local $" = "$s"; #" let us please the color syntax engine
print "#tab";
}
Any idea I can correct the block 2 so that I get the wanted result (the one produced by block 1) ?
It actually does work the same if you assign the same string. Perl's
"\n"
creates a one character string consisting of a newline. With my shell (bash), you'd use
'
'
to do the same.
$ perl a.pl --separator='
'
a
b
ca
b
c
You didn't do this. You passed a string consisting of the two characters \ and n to Perl instead.
If you your program to convert two chars \n into a newline, you'll need to tell it to do so.
my #tab = qw( a b c );
sub handle_escapes {
my ($s) = #_;
$s =~ s/\\([\\a-z])/
$1 eq '\\' ? '\\' :
$1 eq 'n' ? "\n" :
do { warn("Unrecognised escape \\$1"); "\\$1" }
/seg;
return $s;
}
{
my $s = '\n'; #" let us please the color syntax engine
local $" = handle_escapes($s);
print "#tab";
}
{
use Getopt::Long;
my $s;
GetOptions('separator=s' => \$s);
local $" = handle_escapes($s); #" let us please the color syntax engine
print "#tab";
}
$ perl a.pl --separator='\n'
a
b
ca
b
c
I'm running into a little trouble with Perl's built-in split function. I'm creating a script that edits the first line of a CSV file which uses a pipe for column delimitation. Below is the first line:
KEY|H1|H2|H3
However, when I run the script, here is the output I receive:
Col1|Col2|Col3|Col4|Col5|Col6|Col7|Col8|Col9|Col10|Col11|Col12|Col13|
I have a feeling that Perl doesn't like the fact that I use a variable to actually do the split, and in this case, the variable is a pipe. When I replace the variable with an actual pipe, it works perfectly as intended. How could I go about splitting the line properly when using pipe delimitation, even when passing in a variable? Also, as a silly caveat, I don't have permissions to install an external module from CPAN, so I have to stick with built-in functions and modules.
For context, here is the necessary part of my script:
our $opt_h;
our $opt_f;
our $opt_d;
# Get user input - filename and delimiter
getopts("f:d:h");
if (defined($opt_h)) {
&print_help;
exit 0;
}
if (!defined($opt_f)) {
$opt_f = &promptUser("Enter the Source file, for example /qa/data/testdata/prod.csv");
}
if (!defined($opt_d)) {
$opt_d = "\|";
}
my $delimiter = "\|";
my $temp_file = $opt_f;
my #temp_file = split(/\./, $temp_file);
$temp_file = $temp_file[0]."_add-headers.".$temp_file[1];
open(source_file, "<", $opt_f) or die "Err opening $opt_f: $!";
open(temp_file, ">", $temp_file) or die "Error opening $temp_file: $!";
my $source_header = <source_file>;
my #source_header_columns = split(/${delimiter}/, $source_header);
chomp(#source_header_columns);
for (my $i=1; $i<=scalar(#source_header_columns); $i++) {
print temp_file "Col$i";
print temp_file "$delimiter";
}
print temp_file "\n";
while (my $line = <source_file>) {
print temp_file "$line";
}
close(source_file);
close(temp_file);
The first argument to split is a compiled regular expression or a regular expression pattern. If you want to split on text |. You'll need to pass a pattern that matches |.
quotemeta creates a pattern from a string that matches that string.
my $delimiter = '|';
my $delimiter_pat = quotemeta($delimiter);
split $delimiter_pat
Alternatively, quotemeta can be accessed as \Q..\E inside double-quoted strings and the like.
my $delimiter = '|';
split /\Q$delimiter\E/
The \E can even be omitted if it's at the end.
my $delimiter = '|';
split /\Q$delimiter/
I mentioned that split also accepts a compiled regular expression.
my $delimiter = '|';
my $delimiter_re = qr/\Q$delimiter/;
split $delimiter_re
If you don't mind hardcoding the regular expression, that's the same as
my $delimiter_re = qr/\|/;
split $delimiter_re
First, the | isn't special inside doublequotes. Setting $delimiter to just "|" and then making sure it is quoted later would work or possibly setting $delimiter to "\\|" would be ok by itself.
Second, the | is special inside regex so you want to quote it there. The safest way to do that is ask perl to quote your code for you. Use the \Q...\E construct within the regex to mark out data you want quoted.
my #source_header_columns = split(/\Q${delimiter}\E/, $source_header);
see: http://perldoc.perl.org/perlre.html
It seems as all you want to do is count the fields in the header, and print the header. Might I suggest something a bit simpler than using split?
my $str="KEY|H1|H2|H3";
my $count=0;
$str =~ s/\w+/"Col" . ++$count/eg;
print "$str\n";
Works with most any delimeter (except alphanumeric and underscore), it also saves the number of fields in $count, in case you need it later.
Here's another version. This one uses the character class brackets instead, to specify "any character but this", which is just another way of defining a delimeter. You can specify delimeter from the command-line. You can use your getopts as well, but I just used a simple shift.
my $d = shift || '[^|]';
if ( $d !~ /^\[/ ) {
$d = '[^' . $d . ']';
}
my $str="KEY|H1|H2|H3";
my $count=0;
$str =~ s/$d+/"Col" . ++$count/eg;
print "$str\n";
By using the brackets, you do not need to worry about escaping metacharacters.
#!/usr/bin/perl
use Data::Dumper;
use strict;
my $delimeter="\\|";
my $string="A|B|C|DD|E";
my #arr=split(/$delimeter/,$string);
print Dumper(#arr)."\n";
output:
$VAR1 = 'A';
$VAR2 = 'B';
$VAR3 = 'C';
$VAR4 = 'DD';
$VAR5 = 'E';
seems you need define delimeter as \\|
In Perl, what is the difference between ' and " ?
For example, I have 2 variables like below:
$var1 = '\(';
$var2 = "\(";
$res1 = ($matchStr =~ m/$var1/);
$res2 = ($matchStr =~ m/$var2/);
The $res2 statement complains that Unmatched ( before HERE mark in regex m.
Double quotes use variable expansion. Single quotes don't
In a double quoted string you need to escape certain characters to stop them being interpreted differently. In a single quoted string you don't (except for a backslash if it is the final character in the string)
my $var1 = 'Hello';
my $var2 = "$var1";
my $var3 = '$var1';
print $var2;
print "\n";
print $var3;
print "\n";
This will output
Hello
$var1
Perl Monks has a pretty good explanation of this here
' will not resolve variables and escapes
" will resolve variables, and escape characters.
If you want to store your \ character in the string in $var2, use "\\("
Double quotation marks interpret, and single quotation do not
If you are going to create regex strings you should really be using the qr// quote-like operator:
my $matchStr = "(";
my $var1 = qr/\(/;
my $res1 = ($matchStr =~ m/$var1/);
It creates a compiled regex that is much faster than just using a variable containing string. It also will return a string if not used in a regex context, so you can say things like
print "$var1\n"; #prints (?-xism:\()
Perl takes the single-quoted strings 'as is' and interpolates the double-quoted strings. Interpolate means, that it substitutes variables with variable values, and also understands escaped characters. So, your "\(" is interpreted as '(', and your regexp becomes m/(/, this is why Perl complains.
"" Supports variable interpolation and escaping. so inside "\(" \ escapes (
Where as ' ' does not support either. So '\(' is literally \(