Using Java to find substring of a bigger string using Regular Expression

You should be able to use non-greedy quantifiers, specifically *?. You're going to probably want the following:

Pattern MY_PATTERN = Pattern.compile("\\[(.*?)\\]");

This will give you a pattern that will match your string and put the text within the square brackets in the first group. Have a look at the Pattern API Documentation for more information.

To extract the string, you could use something like the following:

Matcher m = MY_PATTERN.matcher("FOO[BAR]");
while (m.find()) {
    String s = m.group(1);
    // s now contains "BAR"
}

the non-regex way:

String input = "FOO[BAR]", extracted;
extracted = input.substring(input.indexOf("["),input.indexOf("]"));

alternatively, for slightly better performance/memory usage (thanks Hosam):

String input = "FOO[BAR]", extracted;
extracted = input.substring(input.indexOf('['),input.lastIndexOf(']'));