Always the same pattern? (anythingTillAnSpace, two digits, one char, two digits, spacePlusAnything)
OK, we try...
xxx 35A44 xxx
xxx A35B44 xxx
match: (.+) (\d\d)(\w)(\d\d) (.+)
replace: \1 \3\2B\4 \5
Match the different groups (digits, char, digits) and catch them in (...)-groups which can reorder in the replacement.
(.*) match anything till an space => xxx , and can be re-used with \1
(\d\d) match two digits => 35, and can be re-used with \2
(\w) match one "word" sign => A, and can be used by \3
Add an "B" at your own, then add the last match by \4 and \5
(Yes, \4 and \5 could be one matching group only too)
match: (.+) (\d\d)(\w)(\d\d .+)
replace: \1 \3\2B\4
or even like
match: (.+ )(\d\d)(\w)(\d\d .+)
There are many ways.
Read an regular expressions tutorial what that all means.