How to insert a character in a string with condition?

Question

Considering we have a string:

str = "1a 789 4/7 123/7 asff %$#7 478 9/4"

How can I insert "/1" for the numbers that are integers, to have the following result:

res = "1a 789/1 4/7 123/7 asff %$#7 478/1 9/4"

Thanks

Answer 1

If there is always one space at a time as shown, or if the number of spaces is irrelevant:

str = "1a 789 4/7 123/7 asff %$#7 478 9/4";

StringSplit[str] /. 
  s_String /; StringMatchQ[s, DigitCharacter ..] :> s <> "/1" // StringRiffle

"1a 789/1 4/7 123/7 asff %$#7 478/1 9/4"

Or adjusting J.M.'s comment code to work:

StringReplace[str, 
 w1 : Whitespace | StartOfString ~~ x : DigitCharacter .. ~~ 
   w2 : Whitespace | EndOfString :> w1 <> x <> "/1" <> w2]

"1a 789/1 4/7 123/7 asff %$#7 478/1 9/4"