sed - replace match containing two wildcards - sed

Question

I'm having trouble trying to replace a match. My example file:
cat ntpd:
OPTIONS="-g"
OPTIONS="-u ntg:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u root:root -p /var/run/ntpd.pid -g"
OPTIONS="-u root:root"
OPTIONS="-u"
I'd like to replace any occurrence of "-u *:*" with "-u ntp:ntp"
I've tried variants of:
sed 's/-u\ \(.*\)\:\(.*\)/-u ntp:ntp/g' ntpd
but that's giving me results in which everything after the replacement string is being lost:
OPTIONS="-g"
OPTIONS="-u ntp:ntp
OPTIONS="-u ntp:ntp
OPTIONS="-u ntp:ntp
OPTIONS="-u"
....if I add a space after the second wildcard (.*\) like sed 's/-u\ \(.*\)\:\(.*\) /-u ntp:ntp/g' ntpd that strangely results in:
OPTIONS="-g"
OPTIONS="-u ntp:ntp-g"
OPTIONS="-u ntp:ntp-g"
OPTIONS="-u root:root"
OPTIONS="-u"
...where a line in which the pattern match no longer matches because it isn't trailed by a space no longer gets replaced (not good), and the line containing a space and more text after is finding the -g as the only remaining text, but I'd assume it would have left all -p /var/run/ntpd.pid -g trailing text.
Any sed experts out there, any/all guidance is appreciated. My desired results would be:
OPTIONS="-g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp"
OPTIONS="-u"

Answer 1

This command should do it:
sed 's/-u\( [^:]\{1,\}:[^ ]\{1,\}\)/-u ntp:ntp/' file
Output:
OPTIONS="-g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u"
Btw, using extended posix regular expressions with -r the command becomes a bit more readable:
sed -r 's/-u( [^:]{1,}:[^ ]{1,})/-u ntp:ntp/' file

Answer 2

This might work for you (GNU sed):
sed -r 's/-u\s\S+:\S+/-u ntp:ntp/' file
Using \s to represent a space and \S to represent a non-space and the qualifier + to represent one or more of the preceeding character or class of characters, the regexp matches a -u followed by a single space, followed by one or more non-spaces, followed by a :, followed by one or more non-spaces.

Answer 3

All proposed answers work to a degree, but both have problems (a trailing " is wrongly dropped).
OPTIONS="-g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp
OPTIONS="-u"
Whereas, the comment provided answer by #melpomene was the first response and ultimate solution, for this case at least.
Using sed 's/-u\ [a-zA-Z0-9_-]*:[a-zA-Z0-9_-]*/-u ntp:ntp/g' ntpd
results in the much desired:
OPTIONS="-g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"
OPTIONS="-u ntp:ntp"
OPTIONS="-u"