s/(".*?")/($x=$1)=~y: :_:, $x/ge
[download]

The regex to match quoted strings is discussed in detail in Jeffrey Friedl's excellent book: Mastering Regular Expressions. The regex to do this (and many more) is also available in the superb Regexp::Common CPAN module.

Modifying ambrus's solution with Friedl's optimized regex to match a quoted string (which also allows for escaped characters) gives:

s/("[^"\\]*(?:\\.[^"\\]*)*")/($x=$1)=~y: :_:, $x/ge;
[download]

use strict;
use warnings;

my ($quote_on,$output);

$_ = q/this is a "test of some regexp" blah blah
this is another "test of some regexp" foo bar/;

for (split //) {
  $quote_on = ($quote_on ? 0 : 1) if (/\"/);
  s/\s/_/g if $quote_on;
  $output .= $_;
}

print "output: $output\n";
[download]

YAPE::Regex::Explain will help you:

The regular expression:

(?-imsx:s/(".*?")/($x=$1)=~y: :_:, $x/ge)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  s/                       's/'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
    x=                       'x='
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
    1                        '1'
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  =~y: :_:,                '=~y: :_:, '
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  x/ge                     'x/ge'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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HTH,




Update: See below for corrections. (Thanks, japhy!)

planetscape

s/(".*?")/($x=$1)=~y: :_:, $x/ge)

[CUT]

  =~y: :_:,                '=~y: :_:, '
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  x/ge                     'x/ge'
[download]

Hm... Is it good explanation??
I'll try in my poor english:

s/(".*?")/($x=$1)=~y: :_:, $x/ge)
^^^^^^^^^1
          ^^^^^^^2
                  ^^^^^^^3
                          ^^4
                             ^^^5
[download]

1) take ALL between " and "
2) So you have ALL in $1. $1 is read only, so copy $1 to $x. And now, for one moment forget about all and work only on $x (this is thanx /e which "wraps an eval{...} around the replacement string and the evaluated result is substituted for the matched substring".
3)in $x replace all ' ' to '_'
4)put replaced $x into first s/from/to/
5)g - works globaly; e - eval the replacment string

greets
Uksza

To explain the first response syntax:

#declarations
my ($quote_on,$output);

#default scalar is the data
$_ = q/this is a "test of some regexp" blah blah
this is another "test of some regexp" foo bar/;

#using $_ begin a for loop performing a split on each #element

for (split //) {

#quote_on is a boolean set to true when the increments reach
#a quote (and then off again when reaching another quote)

 $quote_on = ($quote_on ? 0 : 1) if (/\"/);

#if processing elements after a quote, then replace the
#whitespace with an underscore

 s/\s/_/g if $quote_on;

#assign all that to $output

  $output .= $_;
}
[download]

I didn't write the original post, but I was thinking about doing it a different way, although this one seems to be
perfectly adequate and more importantly, maintainable, than a seemingly more elegant one-liner. Just IMHO