sed replace all tabs and spaces with a single space
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.
+ 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
). 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 -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
[:blank:] character class comprises both spaces and tabs.