static public String allow(String x, String allowChars, char replaceChar) {
    boolean ok = true;
    for (int pos = 0; pos < x.length(); pos++) {
      char c = x.charAt(pos);
      if (!(Character.isLetterOrDigit(c) || (0 <= allowChars.indexOf(c)))) {
        ok = false;
        break;
      }
    }
    if (ok)
      return x;

    // gotta do it
    StringBuilder sb = new StringBuilder(x);
    for (int pos = 0; pos < sb.length(); pos++) {
      char c = sb.charAt(pos);
      if (Character.isLetterOrDigit(c) || (0 <= allowChars.indexOf(c))) {
        continue;
      }

      sb.setCharAt(pos, replaceChar);
    }

    return sb.toString();
  }
static public String allow(String x, String allowChars, char replaceChar) {
    boolean ok = true;
    for (int pos = 0; pos < x.length(); pos++) {
      char c = x.charAt(pos);
      if (!(Character.isLetterOrDigit(c) || (0 <= allowChars.indexOf(c)))) {
        ok = false;
        break;
      }
    }
    if (ok)
      return x;

    
    StringBuilder sb = new StringBuilder(x);
    for (int pos = 0; pos < sb.length(); pos++) {
      char c = sb.charAt(pos);
      if (Character.isLetterOrDigit(c) || (0 <= allowChars.indexOf(c))) {
        continue;
      }

      sb.setCharAt(pos, replaceChar);
    }

    return sb.toString();
  }
static public String allow ( String x , String allowChars , char replaceChar ) { boolean ok = true ; for ( int pos = 0 ; pos < x . length ( ) ; pos ++ ) { char c = x . charAt ( pos ) ; if ( ! ( Character . isLetterOrDigit ( c ) || ( 0 <= allowChars . indexOf ( c ) ) ) ) { ok = false ; break ; } } if ( ok ) return x ; // gotta do it
 StringBuilder sb = new StringBuilder ( x ) ; for ( int pos = 0 ; pos < sb . length ( ) ; pos ++ ) { char c = sb . charAt ( pos ) ; if ( Character . isLetterOrDigit ( c ) || ( 0 <= allowChars . indexOf ( c ) ) ) { continue ; } sb . setCharAt ( pos , replaceChar ) ; } return sb . toString ( ) ; }
Replace any char not alphanumeric or in allowChars by replaceChar.

@param x           operate on this string
@param allowChars  these are ok.
@param replaceChar thar char to replace
@return resulting string.
Replace any char not alphanumeric or in allowChars by replaceChar .