1. notes for regular expression

1) syntax tree.

node type in a syntax tree for implementing NFA is described as follow:

symbol	node type
dot(.)	Leaf Node
^	Head Node
$	Tail Node
[]	O-Node, option node.
*	Star Node
+	Star Node
?	Star Node
{}	Star Node
r1r2	Cat Node
r1|r2	Or Node
()	represent a unit, it will be set as an attribute in the tree node.
\1	leaf node, for back reference.
char	leaf node represents normal character.

2) usage.

syntaxs of the regular expression supported by my implementation.

1. all the syntaxs supported are as mentioned in the previous section.
2. for [], support range and negation, [a-f], [2-9], [^abc].
3. for back reference, referencing index starts from 0, this is quite different from what you usually see. eg, given pattern: ((ab)cd), for testing string abcd, we get: \0 == ab, \1 == abcd. so, group capturing works inside out.
4. use {2, 4} to make repetition of the previous unit. eg, (ab){2, 4} means repeating ab 2 to 4 times(inclusive)

2. other notes, TBD