Java Regex Question extract Substring

Question

Hello
I've readt the regex course on http://www.regenechsen.de/regex_de/regex_1_de.html but the regex rules described in the course and its behavior in the "real world" doesn't makes sense. For sample: in the whole string: <INPUT TYPE="Text" name="Input_Vorname">
the matcher should extract only the fieldname so "Input_Vorname" i tried a lot of patterns so this:
"name="(.*?)\"";
"<.*name=\"(.*)\".?>";
"<.*?name=\"(w+)\".*>";
"name=\".*\"";
and so on. But nothing (NOTHING) works. Either it finds anything or nothing. Whats wrong ?
Can somebody declare me what I've made wrong and where my train of thought was gone wrong?
Roland

Answer 1

When you use the matches() method, the regex has to match the entire input, but if you use find(), the Matcher will search for a substring that matches the regex. Here's how you would use it:  String nameRegex = "name=\"(.*?)\"";
  Pattern namePattern = Pattern.compile(nameRegex,Pattern.CASE_INSENSITIVE);
  Matcher nameMatcher = namePattern.matcher(token);
  if (nameMatcher.find()) {
    String fieldName = nameMatcher.group(1);
  }But the main issue is that you're using the wrong method(s) to retrieve the name. The start() and end() methods return the start and end positions of the entire match, but you're only interested in whatever was matched by the subexpression inside the parentheses (round brackets). That's called a capturing group, and groups are numbered according to the order in which they appear, so you should be using start(1) and end(1) instead of start() and end(). Or you can just use group(1), as I've done here, which returns the same thing as your substring() call.
Knowing that, you could go ahead and use matches(), with an appropriate regex:  String nameRegex = "<.*?name=\"(\\w+)\".*?>";
  Pattern namePattern = Pattern.compile(nameRegex,Pattern.CASE_INSENSITIVE);
  Matcher nameMatcher = namePattern.matcher(token);
  if (nameMatcher.matches()) {
    String fieldName = nameMatcher.group(1);
  }