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This article will show only a few example that would be enough for most people:

[b]How to remove multiple white spaces from a string:[/b]
[code]# echo 'some white spaces' | sed 's/ *//g'
somewhitespaces
[/code]

[b]How to replace multiple white spaces from a string:[/b]
[code]# echo 'some white spaces' | sed 's/ */\ /g'
some white spaces
[/code]

Above sed command replaces multiple spaces with a single space. You can adjust it to replace multiple spaces with other characters, like pipe (|):
[code]# echo 'some white spaces' | sed 's/ */\|/g'
some|white|spaces
[/code]

Special attention needs to be paid to the start and end of the lines:
[code]# echo ' some white spaces ' | sed 's/ */\|/g'
|some|white|spaces|
[/code]

Above example contains multiple spaces at the beginning and one space at the end of the line. Sed replaced all with a pipe.
All of the above sed regular expressions in the replaced field (/ */) contain two spaces, followed by a ster (*). Omitting this will mess up your script.