You use a number in curly brackets to indicate how often an expression can output repeated elements.
For example the expression [a-z]{3} can produce the following three-letter tokens:
•iwx
•hxz
•pfm
•ggi
You can produce variable length strings by using digits separated by a comma. This indicates the range of lengths of the tokens.
For example the expression [a-z]{2,4} can produce strings comprised of two, three, or four lowercase English letters. These might include the following tokens: