Regex for all decimal numbers to match English format

You might first replace all the last comma's with a dot, and then replace the rest of the comma's with an empty string.

,(?=\d+[df]?(?!\S))

• , Match literally
• (?= Positive lookahead, assert what is directly to the right is
  • \d+[df]? Match 1+ digits and optional d or f
  • (?!\S) Assert a whitspace boundary to the right
• ) Close lookahead

Example code:

String s = "52,52d\n"
        + "65.00f\n"
        + "123,50\n"
        + "1,234,567,89\n"
        + "1,234,567.89";

System.out.println(s
        .replaceAll(",(?=\\d+[df]?(?!\\S))", ".")
        .replaceAll(",", "")
);

Output

52.52d
65.00f
123.50
1234567.89
1234567.89

See a Java demo

If there is more text that can contain a comma and you don't want to replace those as a side effect, you can also first match all the parts where you want to do the replacements on and for those matches do the replacements.

\b\d+(?:\,\d+)*(?:\.\d+)?[fd]?\b

Regex demo