Regular expressions or parts of a regular expression can be repeated. I specify the number of repetitions with a type of operator called quantifiers. A quantifier has two attributes:
- A lower limit for the number of repetitions is represented by a natural number (i.e. a non-negative integer)
- An upper limit for the number of repetitions is represented either by a natural number or the empty string. The latter means unlimited.
I write the quantifiers inside a pair of braces, with a comma between the lower and upper limit, for example:
- Zero or one repetition: {0,1}
- One, two or three repetitions: {1,3}
- Fourteen or fifteen repetitions: {14,15}
- Two or more repetitions: {2,}
The quantifier {1,1} is an identity operator:
- a{1,1} equals a
Quantifiers are unary, left associative, and has high precedence. Concatenation as well as Alternation have lower precedence. That gives the following rules:
- Concatination: ab{1,2} equals a(b{1,2})
- Concatination: a{1,2}b equals (a{1,2})b
- Alternation: a|b{1,2} equals a|(b{1,2})
- Alternation: a{1,2}|b equals (a{1,2})|b
I’ve got a lot of syntactic sugar in my regular expression jar. A single natural number in a quantifier represents both the lower and upper limit:
- {3} equals {3,3}
I may write Kleene closure — zero or more repetitions — as an asterisk without braces:
- a* equals a{0,}
I may write Positive closure — one or more repetitions — as a plus sign without braces:
- a+ equals a{1,}
I may write the Optional operation — zero or one repetition — as a question mark without braces:
- a? equals a{0,1}
More algebra:
- (a*)* equals a*
- (a+)+ equals a+
- (a?)? equals a?
- (a*)+ equals (a+)* equals a*
- (a*)? equals (a?)* equals a*
- (a+)? equals (a?)+ equals a*
- Kleene closure is Positive closure or nothing: a* equals (a+|)
- Optional a is a or nothing: a? equals (a|)? equals (a|)
- Kleene closure of a or nothing is Kleene closure of a: (a|)* equals a*
- Positive closure of a is a concatenated with Kleene closure of a: a+ equals aa*
met Jonathan Rasmusson in Chicago last summer at Agile2009. He was then struggling with his upcoming book. I asked curiously when he expected the book to be released. Hopefully within a few months, he answered. A few months happened to be almost a year, but now it’s released. The title is 
month I met Ed Burns in Poland at the GeeCON2010 Java conference. He recently released a new version of his JSF book. But, what caught my attention was another book from 2008. During one of his sessions at GeeCON he replayed interviews with people like Rod Johnson, James Gosling and Andy Hunt. Those interviews were originally recorded for the book 
