Text before first match is kept.

Text after last match is kept.

Regular expression must also "match" all characters between first and last match.

3 matches "per loop", so replace() works as wanted, but match() "looks bad".

It's impossible to keep the 2 "Hello" between first and last match.

Only one match "per loop", so match() looks "cool", but replace() finds only 1 parameter instead of 3.

2 matches per loop, so "Hello" and "World" can be swapped.

"Clearer" example to understand how works $1 .. $9, to be used in the second replace(...,...) parameter.