In this chapter, we’ll look at various one-liners for numbering lines and words, and you’ll get to know the $. special variable. You’ll also learn about Perl golfing, a “sport” that involves writing the shortest Perl program to get a task done.

3.1 Number all lines in a file

    perl -pe '$_ = "$. $_"'  file    标上行号

同

    perl -ne 'print "$. $_"'   file 

3.2 Number only non-empty lines in a file

    perl -pe '$_ = ++$x." $_" if /./'  

同

    perl -pe '$_ = ++$x." $_" if /\S/'  跳过空行标号

3.3 Number and print only non-empty lines in a file

(drop empty lines)

    perl -ne 'print ++$x." $_" if /./'  file

图片.png

3.4 Number all lines but print line numbers only for

non-empty lines

    perl -pe '$_ = "$. $_" if /./' 

图片.png

3.5 Number only lines that match a pattern;

print others unmodified

      perl -pe '$_ = ++$x." $_" if /record/'

图片.png

3.6 Number and print only lines that match a pattern

    perl -ne 'print ++$x." $_" if /regex/' 

图片.png

3.7 Number all lines but print line numbers only for

lines that match a pattern

   perl -pe '$_ = "$. $_" if /regex/' 

图片.png

3.8 Number all lines in a file using a custom format

    perl -ne 'printf "%-5d %s", $., $_'

图片.png

3.9 Print the total number of lines in a file(emulate wc -l)

      perl -lne 'END { print $. }' 

You can do the same thing with this one-liner:

    perl -le 'print $n = () = <>' 

What’s really happening here is the = operator is right-associative, meaning the = on the right is done first and the = on the left is done second:

   perl -le 'print $n = (() = <>)'   

You can also drop the variable $n from this one-liner and force the
scalar context through the scalar operator:

      perl -le 'print scalar(() = <>)' 

And now for a more obvious version:

    perl -le 'print scalar(@foo = <>)'

And here’s another way to do it:

      perl -ne '}{print $.'

3.10 Print the number of non-empty lines in a file

      perl -le 'print scalar(grep { /./ } <>)' 

Some Perl programmers like to create the shortest Perl program that does some particular task—an exercise called Perl golfing. A golfer’s version of this one-liner would replace scalar() with ~~ (double bitwise
negate) and drop the spaces, shortening it like this:

          perl -le 'print ~~grep{/./}<>'  

3.11 Print the number of empty lines in a file

    perl -lne '$x++ if /^$/; END { print $x+0 }'

An alternative to $x+0 is the int operator:

    perl -lne '$x++ if /^$/; END { print int $x }'

You could also modify the previous one-liner by doing this:

    perl -le 'print scalar(grep { /^$/ } <>)'

Or write it with ~~:

    perl -le 'print ~~grep{ /^$/ } <>'

The ~~ does bitwise negation twice, which makes grep execute in the scalar context and return the number of empty lines.

3.12 Print the number of lines in a file that match a

pattern (emulate grep -c)

    perl -lne '$x++ if /regex/; END { print $x+0 }'

3.13 Number words across all lines

    perl -pe 's/(\w+)/++$i.".$1"/ge'

This one-liner uses the /e flag, which makes Perl evaluate the replace part of the s/regex/replace/ expression as code!

3.14 Number words on each individual line

    perl -pe '$i=0; s/(\w+)/++$i.".$1"/ge' 

3.15 Replace all words with their numeric positions

      perl -pe 's/(\w+)/++$i/ge'