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Regular Expressions in JS

When it comes to regular expressions you often hear the sentence:
"If you have a problem and you try to solve it with regular expressions - now you have two problems".

This is probably due to the fact that Regex can seem a little complicated at first. Regex also don't show you error messages when you get something wrong.

But in my opinion there are a lot of usecases where regular expressions come in very handy!

For example when validating input:

Or when trying to extract multiple occurences of something from a string:

(The 2nd argument for the RegExp() constructor is a string of flags that you can set. In the following example i used "g" -global -return multiple occurences of match, "m" -multiline -treat every new line like it is the start of a new string, and "i" -case insensitive -ignore case when matching)

You can even define conditions for the regular expression to match, for example:

(Only match groups of at least 4 alphabetical characters, that do not have 2 digits in front of them)

There a actually 2 ways in JS to define a regular expression! Either with the RegExp constructor as in the examples above, or directly as an argument for functions that support it.

Examples are:

anyString.match(/[0-9]/g) - returns the extracted matching characters

anyString.replace(/A-Z/gi, "*") - returns a new string where the matching characters are replaced by the string in the 2nd argument

As you may have noticed, when declaring a regular expression with the constructor, special characters like \n, \s ect. need to be double escaped (\\) because they are inside if a string. Another difference is: When passing a regex as an argument to a function, you just pass it between two forward slashes (//)

Here is a small overview of what can be matched:

[0-9] -> matches any digit (same as \d)
[A-Za-z] -> machtes any letter
\D -> matches any non digit character
\w -> matches any alphanumeric character and _
\W -> matches any non alphanumeric character and non _
\s -> matches any whitespace character
. -> matches any character

Quantifiers:

? -> 0 or 1 time
* -> 0 or more times
+ -> 1 or more times
{2,4} -> Between 2 and 4 times
{3,} -> minimum 3 times
{,5} -> maximum 5 times

Lookarounds:

(?=G) -> matches any character if it is followed by an uppercase G (positive lookahead)
(?!\d) -> matches any character if it is not followed by a digit (negative lookahead)
(?<=\s) -> matches any character if a space character is directly before it(positive lookbehind)
(?<![A-Z]) -> matches any character if directly before it is not an uppercase letter

This was just a small overview of regular expressions in javascript. There is really a lot more stuff you can achieve with regex.

I hope you enjoyed reading! :)