sed replace all tabs and spaces with a single space

Use sed -e "s/[[:space:]]\+/ /g"

Here's an explanation:

[   # start of character class

  [:space:]  # The POSIX character class for whitespace characters. It's
             # functionally identical to [ \t\r\n\v\f] which matches a space,
             # tab, carriage return, newline, vertical tab, or form feed. See
             # https://en.wikipedia.org/wiki/Regular_expression#POSIX_character_classes

]   # end of character class

\+  # one or more of the previous item (anything matched in the brackets).

For your replacement, you only want to insert a space. [:space:] won't work there since that's an abbreviation for a character class and the regex engine wouldn't know what character to put there.

The + must be escaped in the regex because with sed's regex engine + is a normal character whereas \+ is a metacharacter for 'one or more'. On page 86 of Mastering Regular Expressions, Jeffrey Friedl mentions in a footnote that ed and grep used escaped parentheses because "Ken Thompson felt regular expressions would be used to work primarily with C code, where needing to match raw parentheses would be more common than backreferencing." I assume that he felt the same way about the plus sign, hence the need to escape it to use it as a metacharacter. It's easy to get tripped up by this.

In sed you'll need to escape +, ?, |, (, and ). or use -r to use extended regex (then it looks like sed -r -e "s/[[:space:]]\+/ /g" or sed -re "s/[[:space:]]\+/ /g"

You can use the -s ("squeeze") option of tr:

$ tr -s '[:blank:]' <<< 'test.de.          1547    IN      SOA     ns1.test.de. dnsmaster.test.de. 2012090701 900 1000 6000 600'
test.de. 1547 IN SOA ns1.test.de. dnsmaster.test.de. 2012090701 900 1000 6000 600

The [:blank:] character class comprises both spaces and tabs.

Here are some interesting methods I found via experiments (using xxd to see tabs).

echo -e \\033c
s=$(echo -e "a\t\tb\t\tc\t\td\t\te\tf")

echo 'original string with tabs:'
echo "$s"
echo "$s" | xxd

echo -e '\nusing: \techo "$s" | tr -s \\\\t " "'
echo "$s" | tr -s \\t " "
echo "$s" | tr -s \\t " " | xxd

echo -e '\nusing: \techo "$s" | sed '"'s/\\\\t/ /g'"
echo "$s" | sed 's/\t\+/ /g'
echo "$s" | sed 's/\t\+/ /g' | xxd

echo -e '\nusing: \techo ${s/ / }'
echo ${s/ / }
echo ${s/ / } | xxd

z=$(echo $s)
echo -e '\nusing: \tz=$(echo $s); echo "$z"'
echo "$z"
echo "$z" | xxd

echo -e '\nusing: \tread s < file.in; echo $s'
read s < file.in
echo $s
echo $s | xxd

echo -e '\nusing: \twhile read s; do echo $s; done'
while read s;
do
  echo $s
done < file.in

I like using the following alias for bash. Building on what others wrote, use sed to search and replace multiple spaces with a single space. This helps get consistent results from cut. At the end, i run it through sed one more time to change space to tab so that it's easier to read.

alias ll='ls -lh | sed "s/ \+/ /g" | cut -f5,9 -d" " | sed "s/ /\t/g"'