This is part 3 in the series on regular expressions programming interview questions and answers. Here are the links to the previous posts:

In this post, we will talk about Groups. Groups in regular expressions allows you to perform different operations such as alternations, sub patterns, quantifiers, etc.

**Question: Find all the occurrences of the, The and THE**.

Here we will utilize the notion of groups and alternations. Alternation gives you a choice of alternate patterns to match. Let’s try a few solutions to this problem.

- The simplest solution is
**(the|The|THE)**. This matches any of the 3 different alternatives in the group. - RegEx also has the notion of options. Options let you specify the way you would like to search for a pattern. For our example, we are interested in the ignore case option -
**(?i)**. Using this option, we can solve the above problem as**(?i)the**. - A solution utilizing sub patterns can look like
**(tT)(hH)(eE)**.

**Question: Find all the even numbers between 0 and 99**.

The solution requires you to think and use two alternates – one for 0 to 9 set and the other for remaining using grouping. One possible solution is **\b[24680]\b|\b[1-9][24680]\b**.

**Question: Identify hexadecimal numbers in a string of numbers**.

This one is simple. **[a-fA-F0-9]**.

**Question: Ignore all vowels in the given text**.

In this question, you need to use the negation operator ^. Note that the caret (^) at the beginning of the class means “No, I don’t want these characters.” (The caret must appear at the beginning.). Given this knowledge, the answer is quite simple – **[^aeiou]**

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