Pattern p = Pattern.compile("[a-z]");
Matcher m = p.matcher("toto");
System.out.println(m.matches());

public static boolean isPatternValid(String pattern, String string){
    Pattern p = Pattern.compile(pattern);
    Matcher m = p.matcher(string);
    return m.matches();
}

public static String patternExtension(String extension){
    return "([^\s]+(\.(?i)("+extension+"))$)";
}

Pattern p = Pattern.compile("[a-z]+");

[a-z]+

[a-z]*