Shell Programming and Scripting

It's not weird; that's what happens when you depend on implementation-specific behavior. Three different implementations yield three different results:

Code:

# The OP's HP-UX sed:
echo "a|b|c|d|e" |sed 's/|/ /3g'
a b c d e

# An old OSX/FreeBSD sed
echo "a|b|c|d|e" |sed 's/|/ /3g'
a|b|c d|e

# An old GNU sed
echo "a|b|c|d|e" |sed 's/|/ /3g'
a|b|c d e

The POSIX standard states that combining the global flag with a numeric flag yields undefined behavior. If you want your sed script to be portable to other sed implementations, do not do that. Otherwise, you're probably restricted to running it on your specific sed (although it's possible that there may be other implementations which behave similarly).

Regards,
Alister