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Optional Items

The question mark makes the preceding token in the regular expression optional. E.g.: colou?r matches both colour and color.

You can make several tokens optional by grouping them together using round brackets, and placing the question mark after the closing bracket. E.g.: Nov(ember)? will match Nov and November.

You can write a regular expression that matches many alternatives by including more than one question mark. Feb(ruary)? 23(rd)? matches February 23rd, February 23, Feb 23rd and Feb 23.

Important Regex Concept: Greediness

With the question mark, I have introduced the first metacharacter that is greedy. The question mark gives the regex engine two choices: try to match the part the question mark applies to, or do not try to match it. The engine will always try to match that part. Only if this causes the entire regular expression to fail, will the engine try ignoring the part the question mark applies to.

The effect is that if you apply the regex Feb 23(rd)? to the string Today is Feb 23rd, 2003, the match will always be Feb 23rd and not Feb 23. You can make the question mark lazy (i.e. turn off the greediness) by putting a second question mark after the first.

I will say a lot more about greediness when discussing the other repetition operators.

Looking Inside The Regex Engine

Let's apply the regular expression colou?r to the string The colonel likes the color green.

The first token in the regex is the literal c. The first position where it matches successfully is the c in colonel. The engine continues, and finds that o matches o, l matches l and another o matches o. Then the engine checks whether u matches n. This fails. However, the question mark tells the regex engine that failing to match u is acceptable. Therefore, the engine will skip ahead to the next regex token: r. But this fails to match n as well. Now, the engine can only conclude that the entire regular expression cannot be matched starting at the c in colonel. Therefore, the engine starts again trying to match c to the first o in colonel.

After a series of failures, c will match with the c in color, and o, l and o match the following characters. Now the engine checks whether u matches r. This fails. Again: no problem. The question mark allows the engine to continue with r. This matches r and the engine reports that the regex successfully matched color in our string.

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