Introduction to Regular Expressions
Quantifiers - Optional

You are working on a research project that summarizes the findings of primate behavioral scientists from around the world. Of particular interest to you are the scientists’ observations of humor in chimpanzees, so you whip up some regex to find all occurrences of the word humor in the documents you have collected. To your dismay, your regex misses the observations of amusement written by scientists hailing from British English speaking countries, where the spelling of the word is humour. Optional quantifiers to the rescue!

Optional quantifiers, indicated by the question mark ?, allow us to indicate a character in a regex is optional, or can appear either 0 times or 1 time. For example, the regex humou?r matches the characters humo, then either 0 occurrences or 1 occurrence of the letter u, and finally the letter r. Note the ? only applies to the character directly before it.

With all quantifiers, we can take advantage of grouping to make even more advanced regexes. The regex The monkey ate a (rotten )?banana will completely match both The monkey ate a rotten banana and The monkey ate a banana.

Since the ? is a metacharacter, you need to use the escape character in your regex in order to match a question mark ? in a piece of text. The regex Aren't owl monkeys beautiful\? will thus completely match the text Aren't owl monkeys beautiful?.



The interactive applet in the browser allows you to enter a regular expression and see if it matches a string of text. If a character is matched, you’ll see it highlighted in green. If there’s a checkmark next to the string, you’ve completely matched the whole piece of text!

Enter a regular expression that matches each of the strings listed below “Match these strings” and does NOT match any of the strings listed below “Don’t match these strings.”

When you’ve entered a regular expression that matches the appropriate strings, a key will appear in the browser. Add the key to the end of the URL in the code editor and run the code to unlock the next exercise!

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