How to match whitespace in sed?

The character class \s will match the whitespace characters <tab> and <space>.

For example:

$ sed -e "s/\s\{3,\}/  /g" inputFile

will substitute every sequence of at least 3 whitespaces with two spaces.

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REMARK: For POSIX compliance, use the character class [[:space:]] instead of \s, since the latter is a GNU sed extension. See the POSIX specifications for sed and BREs

This works on MacOS 10.8:

sed -E "s/[[:space:]]+/ /g"

Some older versions of sed may not recognize \s as a white space matching token. In that case you can match a sequence of one or more spaces and tabs with '[XZ][XZ]*' where X is a space and Z is a tab.