2024 © RegexOne
Email |
Twitter
Lesson 7: Mr. Kleene, Mr. Kleene
A powerful concept in regular expressions is the ability to match an arbitrary number of characters. For example, imagine that you wrote a form that has a donation field that takes a numerical value in dollars. A wealthy user may drop by and want to donate $25,000, while a normal user may want to donate $25.
One way to express such a pattern would be to use what is known as the Kleene Star and the Kleene Plus, which essentially represents either 0 or more or 1 or more of the character that it follows (it always follows a character or group). For example, to match the donations above, we can use the pattern \d* to match any number of digits, but a tighter regular expression would be \d+ which ensures that the input string has at least one digit.
These quantifiers can be used with any character or special metacharacters, for example a+ (one or more a's), [abc]+ (one or more of any a, b, or c character) and .* (zero or more of any character).
Below are a few simple strings that you can match using both the star and plus metacharacters.
Exercise 7: Matching repeated characters
| Task | Text | |
| match | aaaabcc |  |
| match | aabbbbc | |
| match | aacc | |
| skip | a | |
| Solution | There are at least two 'a's, zero or more 'b's, and at least one 'c' in each line to match, so you can use the expression 'aa+b*c+' to represent this exactly. Alternatively, an even more restrictive expression would be a{2,4}b{0,4}c{1,2} which puts both an upper and lower bound on the number of each of the characters. |
Solve the above task to continue on to the next problem, or read the Solution.