Everything all about REGEX

 https://towardsdatascience.com/everything-you-need-to-know-about-regular-expressions-8f622fe10b03

Everything you need to know about Regular Expressions

What is a Regular Expression?

On an abstract level a regular expression, regex for short, is a shorthand representation for a set. A set of strings.

Say we have a list of all valid zip codes. Instead of keeping that long and unwieldy list around, it’s often more practical to have a short and precise pattern that completely describes that set. Whenever you want to check whether a string is a valid zip code, you can match it against the pattern. You’ll get a true or false result indicating whether the string belongs to the set of zip codes the regex pattern represents.

Let’s expand on the set of zip codes. A list of zip codes is finite, consists of rather short strings, and is not particularly challenging computationally.

What about the set of strings that end in .csv? Can be quite useful when looking for data files. This set is infinite. You can’t make a list up front. And the only way to test for membership is to go to the end of the string and compare the last four characters. Regular expressions are a way of encoding such patterns in a standardized way.

Regular expressions match sets of strings. Photo by Kristian Strand on Unsplash

The following is a regular expression pattern that represents our set of strings ending in .csv

^.*\.csv$

Let’s leave the mechanics of this particular pattern aside, and look at practicalities: a regex engine can test a pattern against an input string to see if it matches. The above pattern matches foo.csv, but does not match bar.txt or my_csv_file.

Before you use regular expressions in your code, you can test them using an online regex evaluator, and experiment with a friendly UI.

I like regex101.com: you can pick the flavor of the regex engine, and patterns are nicely decomposed for you, so you get a good understanding of what your pattern actually does. Regex patterns can be cryptic.

I’d recommend you open regex101.com in another window or tab and experiment with the examples presented in this article interactively. You’ll get a much better feel for regex patterns this way, I promise.