A regular expression is a formalism used to specify a set of strings with a single expression. Since the implementation of regular expressions comes from the Java standard library, the syntax of expressions is the same as in Java: see http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html.
[p-s]{5}
means the string has to be exactly five characters long and it can only contain the p, q, r and s characters.
[^a-d].*
this example expression matches any string which starts with a character other than a, b, c, d because
the ^ sign means exception;
a-d means characters from a to d;
these characters can be followed by zero or more (*) other characters;
the dot stands for an arbitrary character.
For more detailed explanation of how to use regular expressions, see the Java documentation for java.util.regex.Pattern.
The meaning of regular expressions can be modified using embedded flag expressions. The expressions include the following:
(?i) – Pattern.CASE_INSENSITIVEEnables case-insensitive matching.
(?s) – Pattern.DOTALLIn dotall mode, the dot . matches any character, including line terminators.
(?m) – Pattern.MULTILINEIn multiline mode, you can use ^ and $ to express the beginning and end of the line, respectively (this includes at the beginning and end of the entire expression).
Further reading and description of other flags can be found at http://docs.oracle.com/javase/tutorial/essential/regex/pattern.html.