how to put a dash before the last word in a title?

i have a title as this

27 hello dear inh253686.mp3

i want to put a dash before the last word starting with the letter i

how can i do that?

please refer to post number 51568. it gave me a direction like this.
$regexp(%title%,(.) (.),$1 - $2)

i tried this but failed to get the desired result. i get two dashes instead of one.

action>replace with regular expression
field> title
re>(.) (.)
replace with>$1 - $2)

kindly help.

$regexp(%title%,'\s(i[^ ]+)$', - $1)