I am searching for the most expedient way to change a character in a specific area of a string.

mstr='1st-title: ANAME-HER : 3520931 place[21-22]'
in this string the hyphen between 21 and 22 should be a dot.
if I applied chrtran() I would end up changing all the hyphens which I do not want to do.

right now all I've been able to accomplish is to locate the error in a portion of the string, but putting it back together is starting to get a bit unwieldy.
Is there a simpler more direct route of directing the chrtran() to the area beyond '['

thanks
k