What is the Regex for decimal numbers in Java?

I am not quite sure of what is the correct regex for the period in Java. Here are some of my attempts. Sadly, they all meant any character.

String regex = "[0-9]*[.]?[0-9]*";
String regex = "[0-9]*['.']?[0-9]*";
String regex = "[0-9]*["."]?[0-9]*";
String regex = "[0-9]*[\.]?[0-9]*";
String regex = "[0-9]*[\\.]?[0-9]*";
String regex = "[0-9]*.?[0-9]*";
String regex = "[0-9]*\.?[0-9]*";
String regex = "[0-9]*\\.?[0-9]*";

But what I want is the actual "." character itself. Anyone have an idea?

What I'm trying to do actually is to write out the regex for a non-negative real number (decimals allowed). So the possibilities are: 12.2, 3.7, 2., 0.3, .89, 19

String regex = "[0-9]*['.']?[0-9]*";
Pattern pattern = Pattern.compile(regex);

String x = "5p4";
Matcher matcher = pattern.matcher(x);
System.out.println(matcher.find());

The last line is supposed to print false but prints true anyway. I think my regex is wrong though.