1 | (expression) - expression treated as a unit |
2 | (a, b) - sequence: a followed by b |
3 | (a|b) - choice: a or b but not both |
4 | a? - a or nothing |
5 | a+ - one or more occurrences of a |
6 | a* - zero or more occurrences of a |
7 | Example: |
8 | (title,author+,date,keywords?)* |