UPDATE someTable
SET someCol = regexp_replace(someCol,'(m[0-9]+)M[ ]+m([a-z]+)M','12','gi')
WHERE someCol ~* 'm[0-9]+M[ ]+m[a-z]+M'

m                               Beginning of word
  ([0-9]+)                       1 or more numbers, saved
          M                     End of word
            [ ]+                 1 or more spaces
                m               Beginning of word
                  ([a-z]+)       1 or more letters, saved
                          M     End of word

12 A -> 12A
1A B -> 1A B
A1 B -> A1 B
12 3 -> 12 3
12 A3 -> 12 A3

# In ArcGIS Field Calculator
# split address field
addresslist = address.split()

# check if last element is an alphabetic character(s) only - no numbers
if addresslist[-1:].isalpha():
    # build new address here from address list per your requirements
    # this should get you started. Post back if you need help with this part.

def RemoveWhiteAfterNumber(addressField):
     a = addressField
     for i in range(10):
         a = a.replace( str(i) +" ", str(i))
     return a




RemoveWhiteAfterNumber(!addressField!)