I have a string like "ven|ven|vett|vejj|ven|ven". Treat each "|" delimiter for each column.
By splitting the string with "|" saving all the columns in array and reading each column into $str
So, I'm trying to do this as
$string =~ s/$str/venky/g if $str =~ /ven/i; # it will do globally.
Which not met the requirement.
On-demand basis, I need to replace string at the particular number of occurrence of the string.
For example, I've a request to change 2nd occurrence of "ven" to venky.
Then how can I met this requirement simply? Is it some-thing like
$string =~ s/ven/venky/2;
As of my knowledge we have 'o' for replace once and 'g' for globally. I'm struggling for the solution to get the replacement at particular occurrence. And I should not use pos() to get the position, because string keeps on change. It becomes difficult to trace it every-time. That's my intention.
Please help me on this regard.
There is no flag that you can add to the regex that will do this.
The easiest way would be to split and loop. However, if you insist to use one regex, it is doable:
/^(?:[^v]|v[^e]|ve[^n])*ven(?:[^v]|v[^e]|ve[^n])*\Kven/
If you want to replace the Nth occurrence instead of the second, you can do:
/^(?:(?:[^v]|v[^e]|ve[^n])*ven){N-1}(?:[^v]|v[^e]|ve[^n])*\Kven/
The general idea:
(?:[^v]|v[^e]|ve[^n])* - matches any string that isn't part of ven
\K is a cool matcher that drops everything matched so far, so you can sort of use it as a lookbehind with variable length
Currently you're replacing every instance of'ven' with 'venky' if your string contains a match for ven, which of course it does.
What I assume you're trying to do is to substitute 'ven' for 'venky' within your string if it's the second element:
my $string = 'ven|ven|vett|vejj|ven|ven';
my #elements = split(/\|/, $string);
my $count;
foreach (#elements){
$count++;
s/$_/venky/g if /ven/i and $count == 2;
}
print join('|', #elements);
print "\n";
Your approach was already pretty good. What you described makes sense, but I think you are having trouble implementing it.
I created a function to do the work. It takes 4 arguments:
$string is the string we want to work on
$n is the nth occurance you want to replace
$needle is the thing you want to replace – thing needle in a haystack
Note that right now we allow to pass stuff that might contain regular expressions. So you would have to use quotemeta on it or match with /\Q$needle\E/
$replacement is the replacement for the $needle
The idea is to split up the string, then check each element if it matches the pattern ($needle) and keep track of how many have matched. If the nth one is reached, replace it and stop processing. Then put the string back together.
use strict;
use warnings;
use feature 'say';
say replace_nth_occurance("ven|ven|vett|vejj|ven|ven", 2, 'ven', 'venky');
sub replace_nth_occurance {
my ($string, $n, $needle, $replacement) = #_;
# take the string appart
my #elements = split /\|/, $string;
my $count = 0; # keep track of ...
foreach my $e (#elements) {
$count++ if $e =~ m/$needle/; # ... how many matches we've found
if ($count == $n) {
$e =~ s/$needle/$replacement/; # replace
last; # and stop processing
}
}
# put it back into the pipe-separated format
return join '|', #elements;
}
Output:
ven|venky|vett|vejj|ven|ven
To replace the n'th occurrence of "ven" to "venky":
my $n = 3;
my $test = "seven given ravens";
$test =~ s/ven/--$n == 0 ? "venky" : $&/eg;
This uses the ability with the /e flag to specify the substitution part as an expression.
How do you create a $scalar from the result of a regex match?
Is there any way that once the script has matched the regex that it can be assigned to a variable so it can be used later on, outside of the block.
IE. If $regex_result = blah blah then do something.
I understand that I should make the regex as non-greedy as possible.
#!/usr/bin/perl
use strict;
use warnings;
# use diagnostics;
use Win32::OLE;
use Win32::OLE::Const 'Microsoft Outlook';
my #Qmail;
my $regex = "^\\s\*owner \#";
my $sentence = $regex =~ "/^\\s\*owner \#/";
my $outlook = Win32::OLE->new('Outlook.Application')
or warn "Failed Opening Outlook.";
my $namespace = $outlook->GetNamespace("MAPI");
my $folder = $namespace->Folders("test")->Folders("Inbox");
my $items = $folder->Items;
foreach my $msg ( $items->in ) {
if ( $msg->{Subject} =~ m/^(.*test alert) / ) {
my $name = $1;
print " processing Email for $name \n";
push #Qmail, $msg->{Body};
}
}
for(#Qmail) {
next unless /$regex|^\s*description/i;
print; # prints what i want ie lines that start with owner and description
}
print $sentence; # prints ^\\s\*offense \ # not lines that start with owner.
One way is to verify a match occurred.
use strict;
use warnings;
my $str = "hello what world";
my $match = 'no match found';
my $what = 'no what found';
if ( $str =~ /hello (what) world/ )
{
$match = $&;
$what = $1;
}
print '$match = ', $match, "\n";
print '$what = ', $what, "\n";
Use Below Perl variables to meet your requirements -
$` = The string preceding whatever was matched by the last pattern match, not counting patterns matched in nested blocks that have been exited already.
$& = Contains the string matched by the last pattern match
$' = The string following whatever was matched by the last pattern match, not counting patterns matched in nested blockes that have been exited already. For example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
The match of a regex is stored in special variables (as well as some more readable variables if you specify the regex to do so and use the /p flag).
For the whole last match you're looking at the $MATCH (or $& for short) variable. This is covered in the manual page perlvar.
So say you wanted to store your last for loop's matches in an array called #matches, you could write the loop (and for some reason I think you meant it to be a foreach loop) as:
my #matches = ();
foreach (#Qmail) {
next unless /$regex|^\s*description/i;
push #matches_in_qmail $MATCH
print;
}
I think you have a problem in your code. I'm not sure of the original intention but looking at these lines:
my $regex = "^\\s\*owner \#";
my $sentence = $regex =~ "/^\s*owner #/";
I'll step through that as:
Assign $regexto the string ^\s*owner #.
Assign $sentence to value of running a match within $regex with the regular expression /^s*owner $/ (which won't match, if it did $sentence will be 1 but since it didn't it's false).
I think. I'm actually not exactly certain what that line will do or was meant to do.
I'm not quite sure what part of the match you want: the captures, or something else. I've written Regexp::Result which you can use to grab all the captures etc. on a successful match, and Regexp::Flow to grab multiple results (including success statuses). If you just want numbered captures, you can also use Data::Munge
You can do the following:
my $str ="hello world";
my ($hello, $world) = $str =~ /(hello)|(what)/;
say "[$_]" for($hello,$world);
As you see $hello contains "hello".
If you have older perl on your system like me, perl 5.18 or earlier, and you use $ $& $' like codequestor's answer above, it will slow down your program.
Instead, you can use your regex pattern with the modifier /p, and then check these 3 variables: ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} for your matching results.
The following lines of comma-separated values contains several consecutive empty fields:
$rawData =
"2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,,Clear\n
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,,,\n"
I want to replace these empty fields with 'N/A' values, which is why I decided to do it via a regex substitution.
I tried this first of all:
$rawdata =~ s/,([,\n])/,N\/A/g; # RELABEL UNAVAILABLE DATA AS 'N/A'
which returned
2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,N/A,Clear\n
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,N/A,,N/A,\n
Not what I wanted. The problem occurs when more than two consecutive commas occur. The regex gobbles up two commas at a time, so it starts at the third comma rather than the second when it rescans the string.
I thought this could be something to do with lookahead vs. lookback assertions, so I tried the following regex out:
$rawdata =~ s/(?<=,)([,\n])|,([,\n])$/,N\/A$1/g; # RELABEL UNAVAILABLE DATA AS 'N/A'
which resulted in:
2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,,N/A,Clear\n
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,N/A,,N/A,,N/A,,N/A\n
That didn't work either. It just shifted the comma-pairings by one.
I know that washing this string through the same regex twice will do it, but that seems crude. Surely, there must be a way to get a single regex substitution to do the job. Any suggestions?
The final string should look like this:
2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,N/A,N/A,Clear\n
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,N/A,,N/A,N/A,N/A,N/A,N/A\n
EDIT: Note that you could open a filehandle to the data string and let readline deal with line endings:
#!/usr/bin/perl
use strict; use warnings;
use autodie;
my $str = <<EO_DATA;
2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,,Clear
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,,,
EO_DATA
open my $str_h, '<', \$str;
while(my $row = <$str_h>) {
chomp $row;
print join(',',
map { length $_ ? $_ : 'N/A'} split /,/, $row, -1
), "\n";
}
Output:
E:\Home> t.pl
2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,N/A,Clear
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,N/A,N/A,N/A,N/A
You can also use:
pos $str -= 1 while $str =~ s{,(,|\n)}{,N/A$1}g;
Explanation: When s/// finds a ,, and replaces it with ,N/A, it has already moved to the character after the last comma. So, it will miss some consecutive commas if you only use
$str =~ s{,(,|\n)}{,N/A$1}g;
Therefore, I used a loop to move pos $str back by a character after each successful substitution.
Now, as #ysth shows:
$str =~ s!,(?=[,\n])!,N/A!g;
would make the while unnecessary.
I couldn't quite make out what you were trying to do in your lookbehind example, but I suspect you are suffering from a precedence error there, and that everything after the lookbehind should be enclosed in a (?: ... ) so the | doesn't avoid doing the lookbehind.
Starting from scratch, what you are trying to do sounds pretty simple: place N/A after a comma if it is followed by another comma or a newline:
s!,(?=[,\n])!,N/A!g;
Example:
my $rawData = "2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,,Clear\n2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,,,\n";
use Data::Dumper;
$Data::Dumper::Useqq = $Data::Dumper::Terse = 1;
print Dumper($rawData);
$rawData =~ s!,(?=[,\n])!,N/A!g;
print Dumper($rawData);
Output:
"2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,,Clear\n2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,,,\n"
"2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,N/A,Clear\n2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,N/A,N/A,N/A,N/A\n"
You could search for
(?<=,)(?=,|$)
and replace that with N/A.
This regex matches the (empty) space between two commas or between a comma and end of line.
The quick and dirty hack version:
my $rawData = "2008-02-06,8:00 AM,14.0,6.0,59,1027,-9999.0,West,6.9,-,N/A,,Clear
2008-02-06,9:00 AM,16,6,40,1028,12,WNW,10.4,,,,\n";
while ($rawData =~ s/,,/,N\/A,/g) {};
print $rawData;
Not the fastest code, but the shortest. It should loop through at max twice.
Not a regex, but not too complicated either:
$string = join ",", map{$_ eq "" ? "N/A" : $_} split (/,/, $string,-1);
The ,-1 is needed at the end to force split to include any empty fields at the end of the string.