Here a little extract out of the possibilities of regular expressions (regular expressions):
^ standing lonely it represents the beginning of a character string
. it represents any sign
+ the previous sign must appear at least once (therefore, .+ represents any character string with at least one sign)
* the previous sign can appear not once to any often (therefore, .* represents any character strings)
[abc] one of the signs in the brackets must appear instead of the bracket-expression („a[12]b“ can match „a1b“ and „a2b“)
[^abc] none of the signs in the brackets is allowed to appear instead of the bracket-expressionkeines („a[^12]b“ cannot match „a1b“ and „a2b“, but „a3b“)
[a-c] one of the signs in the stated area must appear instead of the bracket-expression („a[1-3]b“ matches „a1b“, „a2b“ and „a3b“, but not „a4b“)
[^a-c] none of the signs in the stated area is allowed to appear instead of the bracket-expression
? the precedent sign is allowed to appear once at the most („a?b“ matches „ab“ and „b“, but not „aab“).
\ in front of a special character it makes sure that it is interpreted as a sign. Together with particular letters (\n oder \d) also for special signs.
\d This sign must be a digit.
\D This sign is allowed to be any sign except for a digit.
\w This sign must be of the following amount „0“ to „9“, „A“ to „Z“, „a“ to „z“ to „_“.
If you do not want special characters to be interpreted as control characters, but as normal signs, you must add „\“ as a prefix (in Java-String you have to input a double „\\“).
A little Java-example:
String str = "hello.txt";
System.out.println(str.matches("^hello\\..*")); // true; corresponds to a "hello.*"
Detailed information (in engl.) at this page:
http://www.regular-expressions.info