[Solved] No match for Java Regular Expression

Your content contains no " quotes, and no text gm, so why would you expect that regex to match?

FYI: Syntaxes like "foo"gm or /foo/gm are something other languages do for regex literals. Java doesn’t do that.

The g flag is implied by the fact that you’re using a find() loop, and m is the MULTILINE flag that affects ^ and $ and you can specify that using the (?m) pattern, or by adding a second parameter to compile(), i.e. one of these ways:

Pattern p = Pattern.compile("foo", Pattern.MULTILINE);

Pattern p = Pattern.compile("(?m)foo");

Your regex should simply be:

(?m)^.*(?==)

which means: Match everything from the beginning of a line up to the last = sign on the line.

Test

String content = "This is an example.=This is an example\nThis is second:=This is second";
String regex = "(?m)^.*(?==)";
Matcher m = Pattern.compile(regex).matcher(content);
List<String> mKeys = new ArrayList<>();
while (m.find()) {
   mKeys.add(m.group());
}
System.out.println(mKeys);

Output

[This is an example., This is second:]