Regex: deleting non-matching words

[pete]

I have input strings where some words start with an underscore. The plan
is to remove all words that do NOT strt with an underscore and simply
keep the rest. So for example starting with
"word1 word2 _word3 word4 word5 _word6 _word7 word8"

I'm trying to end up with
"_word3 _word6 _word7"

The expression I have got so far is s/.*?(_[a-z0-9]+).*?/ $1/gi;
and my understanding is as follows:
The first ".*?" part removes everything up to the first matching RE
The "(_[a-z0-9]+)" matches any letter/number combination that starts
with an underscore [sidenote: yes, I know: \w+]
The final ".*?" removes everything up to the next match, or up to
the end of the string.

Here's how I have the RE in a program
$_=(<>);
s/.*?(_[a-z0-9]+).*?/ $1/gi;
print "Have: $_";

and here's how I run it:
echo "word1 word2 _word3 word4 word5 _word6 _word7 word8" | perl s.pl

and here's the output I get:
Have: _word3 _word6 _word7 word8

Question: Why didn't "word8" get eaten like all its precedessors? and
what do I have to do to match it for removal.

If you have time, I'm looking for enlightenment more than solutions. I
am obviously missing something crucial, but all the online tutorials
I've found stop short of explaining this sort of thing.

 

[Scott Bryce]

I have input strings where some words start with an underscore. The plan
is to remove all words that do NOT strt with an underscore and simply
keep the rest. So for example starting with
"word1 word2 _word3 word4 word5 _word6 _word7 word8"

I'm trying to end up with
"_word3 _word6 _word7"

The expression I have got so far is s/.*?(_[a-z0-9]+).*?/ $1/gi;
and my understanding is as follows:
The first ".*?" part removes everything up to the first matching RE
The "(_[a-z0-9]+)" matches any letter/number combination that starts
with an underscore [sidenote: yes, I know: \w+]
The final ".*?" removes everything up to the next match, or up to
the end of the string.

Here's how I have the RE in a program
$_=(<>);
s/.*?(_[a-z0-9]+).*?/ $1/gi;
print "Have: $_";

and here's how I run it:
echo "word1 word2 _word3 word4 word5 _word6 _word7 word8" | perl s.pl

and here's the output I get:
Have: _word3 _word6 _word7 word8

Question: Why didn't "word8" get eaten like all its precedessors? and
what do I have to do to match it for removal.

An RE does not remove anything, as you have suggested. An RE matches
something. A substitute replaces whatever is matched with the
replacement string.

After you have matched '_word3', '_word6', and '_word7', nothing else in
the string matches your RE, so no further substitutions are made.

When a string can be split into words, and each word evaluated based on
its first character, I wouldn't use REs.

----------------

use strict;
use warnings;

my $original_string = 'word1 word2 _word3 word4 word5 _word6 _word7 word8';
my @word_list;

for my $word (split ' ', $original_string)
{
push @word_list, $word if index($word, '_') == 0;
}

my $new_string = join ' ', @word_list;
print $new_string;

 

[C.DeRykus]

I have input strings where some words start with an underscore. The plan
is to remove all words that do NOT strt with an underscore and simply
keep the rest. So for example starting with
"word1 word2 _word3 word4 word5 _word6 _word7 word8"

I'm trying to end up with
"_word3 _word6 _word7"

The expression I have got so far is s/.*?(_[a-z0-9]+).*?/ $1/gi;
and my understanding is as follows:
The first ".*?" part removes everything up to the first matching RE
The "(_[a-z0-9]+)" matches any letter/number combination that starts
with an underscore [sidenote: yes, I know: \w+]
The final ".*?" removes everything up to the next match, or up to
the end of the string.

Here's how I have the RE in a program
$_=(<>);
s/.*?(_[a-z0-9]+).*?/ $1/gi;
print "Have: $_";

and here's how I run it:
echo "word1 word2 _word3 word4 word5 _word6 _word7 word8" | perl s.pl

and here's the output I get:
Have:  _word3 _word6 _word7 word8

Question: Why didn't "word8" get eaten like all its precedessors? and
what do I have to do to match it for removal.

If you have time, I'm looking for enlightenment more than solutions. I
am obviously missing something crucial, but all the online tutorials
I've found stop short of explaining this sort of thing.

The problem is that once you've matched a
target substring, ie, _[a-z0-9]+ then the
regex .*? lazily stops as soon as possible
since .*? says match any character 0 or more
times minimally (also termed lazily). So the
regex lazily chooses 0 and completes a match.

That works but then the only glitch is that
the lazy .*? fails to consume the rest of the
string once the final target_word7 is found
and you're left with ' word8'.

One way to fix that:

s/ .*? # match minimally
( _[a-z0-9]+ | $ ) # up to target or eol
/ $1/gix;

Now the regex matches_word7, but then
tries to match one of two alternatives:

Either: _[a-z0-9]+
or: end-of-line

The former isn't found but latter is and
the rest of the string is consumed up to
the end-of-line just before \n.

 

[pete]

Thanks Charles, Scott and Tad
I do appreciate the time you've all taken to put me right. The big thing
I was missing was the REGEX pointer and how it iterates through input
string. I did think that the trailing ".*?" would match everything after
the last bracketed part of the RE but now I think I understand better
why it doesn't.

I have to admit, I've never used Perl's grep - though I use the Unix/Linux
versions frequently. Looks like I have a new tool to play with!

Pete