a pair of square brackets ([]) matches any character in the brackets


opening = "this is a test\nthis is a test,\n"
opening.grep(/t[hi]s/)

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

a pair of square brackets ([]) matches any character in the brackets : grep « String « Ruby