Braces ({}) specifies the exact number of digits, such as \d{3} or \d{4}


phone = "(555)123-4567"
phone.grep(/[\(\d{3}\)]?\d{3}-\d{4}/)# => ["(555)123-4567"]

1.	Regular Expressions
2.	match the first line just by using a word in the pattern
3.	a pair of square brackets ([]) matches any character in the brackets
4.	match alternate forms of a pattern using the pipe character (\|)
5.	Grouping uses parentheses to group a subexpression, like this one that contains an alternation
6.	Anchors anchor a pattern to the beginning (^) or end ($) of a line
7.	\d represents a digit; it is the same as using [0-9].
8.	Similar to $, the shortcut \z matches the end of a string, not a line
9.	Find out the phone number
10.	? is a repetition operator
11.	The plus sign (+) operator indicates one or more of the previous pattern

Braces ({}) specifies the exact number of digits, such as \d{3} or \d{4} : grep « String « Ruby