You can use the question mark (?) to indicate zero or one occurrences of the preceding element, the asterisk (*) to indicate zero or more occurrences of the preceding element, and the plus sign (+) to indicate one or more occurrences of the preceding element. For example, colou?r matches both "color" and "colour"; "ab*c" matches "ac", "abc", "abbc", etc.; "ab+c" matches "abc", "abbc", "abbbc", etc.
You can use a vertical bar (|) to separate alternatives, such as "gray|grey".
You can use parentheses ("()") to define the scope and precedence of the operators. For example, gr(a|e)y matches "gray" and "grey".