*. zero or more of the preceeding character. a* = blank (empty string), a, aa, aaa,…
TRANSCRIPT
*
zero or more of the preceeding character. A* = blank (empty
string), A, AA, AAA,…
+
1 or more of the preceeding character. B+ = B, BB, BBB, …
?
zero or one of the preceeding char. A? = blank, A
|
OR. A|B = A, B alternative = [ ]
(a+|b+)cc
acc, bcc, aacc, bbcc, GROUPING: gray | grey = gr(a|e)y; @([_a-zA-Z\d\-]+(\.[_a-zA-
A\d\-]+)+)
{a, b, c}
{ } the set of alphabet for a language
\
backslash = escape (the orginal meaning of the char follows it). \. = decimal point, . = any char
[ab]
a or b. [a-z] = a single char between a-z.
^ caret
NOT. ^t = the string cannot contain t
. Period or dot
any character (almost)
- class, inside [ ]
[a-z] matches all 26 letters. [a-zA-Z] matches all 52 letters.
Hyphen
$ the dollar sign
\d
matches a single numeric character, [0-9];
\w
matches a single alphanumeric character = [a..z, A..Z, 0..9] [a-
zA-Z0-9]
\W
matches a non-alphanumeric character = [^\w]
\D
matches a non-numeric character = [^\d]
\s
matches a single space character
\S
matches a single non-space character
[\.\d+]
=.123, or .365, etc
* and +
first, then concatenation, then | (or)
BNF
::=; <non-terminal symbols>; | ; terminal symbol; alphabet
Syntax diagram
non-terminal symbols rect box; terminal symbols oval box.
RPN
Reversed Polish Notation, Postfix notation, simpler for
machine to evaluate. No brackets.
(0|1)*
any order. * first then the |.