20 Regular Expression Interview Questions and Answers

Regular expressions are a powerful tool for developers when it comes to manipulating strings and text. They are often used in text editors and IDEs to search and replace code, or to validate user input. If you’re interviewing for a position that involves coding or working with text, it’s likely that you’ll be asked questions about regular expressions. In this article, we’ll review some of the most common regular expression questions and how you should answer them.

Regular Expression Interview Questions and Answers

Here are 20 commonly asked Regular Expression interview questions and answers to prepare you for your interview:

1. What are regular expressions in Python?

Regular expressions are a powerful tool for matching patterns in strings. In Python, regular expressions are handled by the re module. The re module provides a set of functions that allow you to search for patterns in strings, replace those patterns with other strings, and split strings into a list of substrings.

2. How would you check if a string contains only numbers using a regular expression?

You could use the regular expression ^[0-9]+$ which would check to see if the string only contains numbers and is at least one character long.

3. Can you explain the difference between + and * operators?

The + operator matches one or more occurrences of the character or group that it precedes, while the * operator matches zero or more occurrences of the character or group that it precedes. So, for example, if you have the regular expression “a+b*c”, it will match any string that starts with an “a” and then has any number of b’s (including zero) before ending with a “c”.

4. How do you search for patterns that match 0 or 1 instances of a particular character?

You can use the question mark character, which represents 0 or 1 instances of the character that precedes it. For example, if you wanted to find all instances of the letter “a” followed by either 0 or 1 instances of the letter “b”, you would use the regular expression “ab?”.

5. What is Greedy Matching in Regular Expressions?

Greedy matching is a type of matching that occurs in regular expressions when the quantifier (the symbol that indicates how many times a character can occur) is set to match the maximum number of characters possible. For example, the regular expression .* will match any character, including spaces, zero or more times. This means that it will match as much of the string as possible.

6. Can you explain what non-capturing groups are used for?

Non-capturing groups are used to group together regular expression patterns without capturing the results of the match. This is useful if you want to apply a quantifier to a group of patterns but do not want to capture the results of the match.

7. What is the best way to convert a string into an ArrayList using a single regex pattern?

The best way to convert a string into an ArrayList using a single regex pattern is to use the split() method. This method will take a regular expression as an argument, and will return an array of strings that have been split up by the regex.

8. When should you use raw strings as opposed to ordinary strings?

Raw strings are used when you need to match a regular expression pattern exactly, without any special characters being interpreted as metacharacters. This is often the case when working with backslashes, which are used as escape characters in many programming languages. By using a raw string, you can ensure that the backslashes are not interpreted as escape characters, but are instead matched as part of the regular expression pattern.

9. How can you split a string by multiple delimiters using a regular expression?

You can use the | character to denote multiple delimiters in a regular expression. For example, if you wanted to split a string by either a comma or a space, you could use the following regular expression: ,|\s

10. What’s the difference between re.findall() and re.search() methods?

The re.findall() method returns a list of all matches in the string, while the re.search() method returns a Match object for the first match.

11. How can you avoid matching the same word twice when searching for words separated by whitespace?

You can use the \b metacharacter to match the beginning or end of a word. This will prevent the regular expression from matching the same word twice in a row.

12. What is negative lookahead?

Negative lookahead is a feature of regular expressions that allows you to specify a pattern that should not match the input string. For example, if you were looking for a phone number in a string, you could use negative lookahead to ensure that the string did not contain any letters.

13. How do you find all URLs within a given string?

You can find all URLs within a given string by using a regular expression to search for patterns that match the syntax of a URL. For example, the regular expression “http://[^ ]+” will match any string that begins with “http://” and is followed by one or more non-space characters.

14. What does the $ symbol denote in a regular expression?

The $ symbol is used to denote the end of a regular expression. This can be useful when you are trying to match patterns that occur at the end of a string, for example.

15. What is the difference between .* and .+ in regular expressions?

The .* regular expression matches any character, zero or more times. The .+ regular expression matches any character, one or more times.

16. What are some examples of real world applications of regular expressions?

Real world applications of regular expressions can be found in a variety of places. One common use is in text editors and IDEs, where they are used to perform search and replace operations. They are also commonly used in web development, for tasks such as validating user input and parsing data from HTML or XML documents. Finally, they are also used in many command line tools, such as grep, for searching through files or output.

17. What happens when a group does not participate in the match at all?

In this case, the group simply does not exist in the match. This can happen if the group is optional and there is no match, or if the group is matched but the match fails.

18. What is Group Extraction? Why is it useful?

Group extraction is a process of extracting a certain subpattern from a larger regular expression pattern. This is useful because it allows you to break down a complex regular expression into smaller, more manageable pieces. It also makes it easier to reuse parts of a regular expression pattern.

19. What are backreferences and how are they useful?

Backreferences are a way of referring back to a previously matched group within a regular expression. This can be useful for matching patterns that repeat, or for extracting information from a string. For example, if you are trying to match a phone number in a string, you could use a backreference to match the area code.

20. What are the differences between greedy, reluctant, and possessive quantifiers?

The main difference between these three quantifiers is how they handle backtracking. Backtracking is what happens when the regex engine reaches a point where it is unsure of which path to take.

A greedy quantifier will always take the longest path possible, even if that means backtracking later on.

A reluctant quantifier will take the shortest path possible, even if that means backtracking later on.

A possessive quantifier will never backtrack. Once it has made a match, it will not try any other possibilities even if that means a longer overall match could be found.