With this information, we can match most strings of a known length, but most of the time, we don't know how many characters to match inside a pattern. Regular expressions can take care of this, too. We can modify a pattern by appending one of several hard-to-remember punctuation symbols to match multiple characters.
The asterisk (*) character says that the previous pattern can be matched zero or more times. This probably sounds silly, but it's one of the most useful repetition characters. Before we explore why, consider some silly examples to make sure we understand what it does:
'hello' matches pattern 'hel*o' 'heo' matches pattern 'hel*o' 'helllllo' matches pattern 'hel*o'
So, the * character in the pattern says that the previous pattern (the l character) is optional, and if present, can be repeated as many times as possible to match the pattern. The rest of the characters (h, e, and o) have to appear exactly once.
It's pretty rare to want to match a single letter multiple times, but it gets more interesting if we combine the asterisk with patterns that match multiple characters. So, .*, for example, will match any string, whereas [a-z]* matches any collection of lowercase words, including the empty string. Here are a few examples:
'A string.' matches pattern '[A-Z][a-z]* [a-z]*\.' 'No .' matches pattern '[A-Z][a-z]* [a-z]*\.' '' matches pattern '[a-z]*.*'
The plus (+) sign in a pattern behaves similarly to an asterisk; it states that the previous pattern can be repeated one or more times, but, unlike the asterisk, is not optional. The question mark (?) ensures a pattern shows up exactly zero or one times, but not more. Let's explore some of these by playing with numbers (remember that \d matches the same character class as [0-9]:
'0.4' matches pattern '\d+\.\d+' '1.002' matches pattern '\d+\.\d+' '1.' does not match pattern '\d+\.\d+' '1%' matches pattern '\d?\d%' '99%' matches pattern '\d?\d%' '999%' does not match pattern '\d?\d%'