1. *

2. zero or more of the preceeding character. A* = blank (empty string), A, AA, AAA,…

3. +

4. 1 or more of the preceeding character. B+ = B, BB, BBB, …

5. ?

6. zero or one of the preceeding char. A? = blank, A

7. |

8. OR. A|B = A, B alternative = [ ]

9. (a+|b+)cc

10. acc, bcc, aacc, bbcc, GROUPING: gray | grey = gr(a|e)y; @([_a-zA-Z\d\- ]+(\.[_a-zA-A\d\-]+)+)

11. {a, b, c}

12. { } the set of alphabet for a language

13. \

14. backslash = escape (the orginal meaning of the char follows it). \. = decimal point,. = any char

15. [ab]

16. a or b. [a-z] = a single char between a-z.

17. ^ caret

18. NOT. ^t = the string cannot contain t

19. . Period or dot

20. any character (almost)

21. - class, inside [ ]

22. [a-z] matches all 26 letters. [a- zA-Z] matches all 52 letters. Hyphen

23. $ the dollar sign

25. \d

26. matches a single numeric character, [0-9];

27. \w

28. matches a single alphanumeric character = [a..z, A..Z, 0..9] [a- zA-Z0-9]

29. \W

30. matches a non-alphanumeric character = [^\w]

31. \D

32. matches a non-numeric character = [^\d]

33. \s

34. matches a single space character

35. \S

36. matches a single non-space character

37. [\.\d+]

38. =.123, or.365, etc

39. * and +

40. first, then concatenation, then | (or)

41. BNF

42. ::=; ; | ; terminal symbol; alphabet

43. Syntax diagram

44. non-terminal symbols rect box; terminal symbols oval box.

45. RPN

46. Reversed Polish Notation, Postfix notation, simpler for machine to evaluate. No brackets.

47. (0|1)*

48. any order. * first then the |.