Having trouble splitting words in java

I have the following string:

String text = "stack Overflow, stack"

a char:

char c = "*"

and another string:

String word = "Overflow"

the whole idea is to check for the word "overflow" in the text string, and surround it with the char c. The output should look something like this:

"stack *Overflow*, stack"

First, in order to solve this, i thought of splitting the string into words, and then check for each word if it contains the desired string as a substring of itself. Then create a stringBuilder, and append the char on each end of the word. I get no erros, but a wrong output though:

"stack *Overflow,* stack

Any help on how to solve this properly ?