If you are using patterns or regular expressions then the period is the wildcard for any character and * means 0 or more occurances. The expression (.*[0-9].*) would search for a number anywhere in the string. There are also some other operators which are good to know such as + 1 or more rather than 0 or more. The back of the IS manual has a full section on regular expressions and pattern matching.
I typically try to avoid the LIKE operator in expressions (rules, filters, etc.) and use it only in creating views. If you are using LIKE then % is the wildcard as in SQL. Concatenation uses || rather than + though so don't get yourself in trouble by trying to use a command LIKE '%' + $field + '%';