Simple one-liners: last line missing

Ruby
1

Hi

Let’s say I’d want to modify each line of a file. Here’s the dummy
data:

$ cat data.txt
first foo
second foo
third foo
$

This works:

$ ruby -ne ‘$_ =~ /(\S+)\s+(\S+)/;puts “#{$2} #{$1}”’ data.txt
foo first
foo second
foo third
$

I don’t really need to achieve that modification. Instead I’d like to
understand why the following version one doesn’t work:

$ ruby -pe ‘sub(/(\S+)\s+(\S+)/,"#{$2} #{$1}")’ data.txt

foo first
foo second
$

TIA,
Tobi

2

“T” == Tobi R. [email protected] writes:

T> $ ruby -pe ‘sub(/(\S+)\s+(\S+)/,“#{$2} #{$1}”)’ data.txt

The replacement string is build when #sub is called

for the first call $1 = $2 = nil ==> the replacement string is ’ ’

for the second call, $1 = ‘first’, $2 = ‘foo’ ==> the replacement
string
is ‘foo first’

etc,

Guy Decoux

3

On Tue, 14 Mar 2006 01:46:50 +0900, Tobi R. [email protected]
wrote:
[snip]

I don’t really need to achieve that modification. Instead I’d like to
understand why the following version one doesn’t work:

$ ruby -pe ‘sub(/(\S+)\s+(\S+)/,“#{$2} #{$1}”)’ data.txt

foo first
foo second
$

When doing substitutions with a string replacement, you do not get what
you might expect because the interpolation of #{$1} in the replacement
string happens when the arguments are evaluated and passed into the
sub()
or gsub() methods, not during the match. That means they will have
whatever
value they had from the last successful match (or nil, as in your case
with
the first line printed).

To access backreferences within a replacement string, you may use the
same
notation as you do for backreferences within the pattern itself: \1, \2,
etc. For your example:

ruby -pe ‘sub(/(\S+)\s+(\S+)/,%q(\2 \1))’ data.txt

Alternatively, you may use the block form of replacement, in which case
the
block is evaluated at match-time and $1, $2, etc are available and refer
to
their respective backreferences within the current match:

ruby -pe ‘sub(/(\S+)\s+(\S+)/){“#{$2} #{$1}”}’ data.txt

cheer’s
andrew

4

On Tue 2006-03-14 ts wrote:

etc,

Thanks. I guess my (current) opinion is that it should work.

Tobi

5

I think you want,

ruby -pe ‘sub(/(\S+)\s+(\S+)/,"\2 \1")’ data.txt

6

On Tue 2006-03-14 Tobi R. wrote:

is ‘foo first’

etc,

Thanks. I guess my (current) opinion is that it should work.

Tobi

Now I understand that it can’t work.

Thanks again,
Tobi