SimpleDateFormat RegEx formats

Regex master needed!

I have an variable timestamp coming from the server and I need to find which format is used everytime. I've tried implementing a regex formats but they don't work. I'm fairly new to regex patterns but still I've tried working them up my self or else look for a specific example but couldn't find so I'm asking you.

The formats from the server can look like this:

"2015-02-23 15:27:31 UTC"

or

"2015-01-22T19:38:40Z"

here is the code to find the formats:

private static String getFormat(String time) {
    String firstRegEx = "^\\d{4}-\\d{2}-\\d{2}\'T+\'\\d{2}:\\d{2}:\\d{2}\'Z\'$";
    String secondRegEx = "^\\d{4}-\\d{2}-\\d{2}\\s\\d{2}:\\d{2}:\\d{2}\\s\\w{3}$";

    if (time.toLowerCase().matches(firstRegEx)) {
        return firstRegEx;
    } else if (time.toLowerCase().matches(secondRegEx)) {
        return secondRegEx;
    }
    return null;
}

Can you look at my regex patterns and tell me what am I doing wrong?