Java Regex Matching, 2 simple regex questions

Question

Question 1.

String matchedKey = "sessions.0.something.else";
Pattern newP = Pattern.compile("sessions\\.([^\\.]+)(\\..+)");
m = newP.matcher(matchedKey);

System.out.println(m.group(1)); // has nothing. Why?

sessions\\. // word "sessions" followed by .
([^\\.]+)   // followed by something that is not a literal . at least once
(\\..+)     // followed by literal . and anything at least once

I would have expected for m.group(1) to be 0

Question 2

String mask = "sessions.{env}";
String maskRegex = mask.replace(".", "\\\\.").replace("env", "(.+)")
                                   .replace("{", "").replace("}", "");
// produces mask "sessions\\.(.+))"

When used as

Pattern newP = Pattern.compile("sessions\\.(.+))"); // matches  matchedKey (above)
Pattern newP = Pattern.compile(maskRegex);          // does not match matchedKey (above)

Why is that?

Answer 1

You haven't called Matcher.find() OR Matcher.macthes() method in your both questions.

Use it like this:

if (m.find())
   System.out.println("g1=" + m.group(1));

Also good to check Matcher.groupCount() value.

Answer 2

before you can access the groups of a matcher, you have to call matches on it:

String matchedKey = "sessions.0.something.else";
Pattern newP = Pattern.compile("sessions\\.([^\\.]+)(\\..+)");
m = newP.matcher(matchedKey);
if (m.matches()) {
    System.out.println(m.group(1));
}

find will also do if you want to find the pattern anywhere within the string. matches checks if the whole string matches your pattern from the beginning to the end.