One way you can do it is simply keeping track of brackets with a boolean semaphore
public static String modify(String expression)
{
String temp = expression;
StringBuilder validated = new StringBuilder();
boolean inBrackets=false;
for(int idx=0; idx<temp.length()-1; idx++)
{
if((Character.isLetter(temp.charAt(idx))) && (Character.isLetter(temp.charAt(idx+1))))
{
if(!inBrackets){
inBrackets = true;
validated.append("(");
}
validated.append(temp.substring(idx,idx+1));
validated.append("*");
}
else{
validated.append(temp.substring(idx,idx+1));
if(inBrackets){
validated.append(")");
inBrackets=false;
}
}
}
validated.append(temp.substring(temp.length()-1));
if(inBrackets){
validated.append(")");
inBrackets=false;
}
return validated.toString();
}
Also never use string concatenation instead use StringBuilder or its predecessor StringBuffer in case you are seeking thread safe solution.