Regular Expressions 101

Overview

Regular expressions (or “Regex” … said “Rej-Ex”) are a means of identifying patterns in strings. We commonly use them to validate input, replace character patterns with other characters or refer to character patterns in a string.

Mechanically, a regex is composed of a “Pattern To Match” between 2 delimiters (usually a /), followed by some optional instructions, known as “quantifiers”. Quantifiers are single letter instructions:

g = “Global Match”
i = “Case Insensitive”

e.g. `/foo/ig`

For our examples, lets use the following string:

The rain in Spain falls mainly in the plain

Does this string contain the letter “T”?

/T/
/t/i

We can use a “Range”, which is a group of letters or numbers in brackets to see if there are things of the same basic type

Does this string contain any capital letters?

/[A-Z]/

Lowercase letters?

/[a-z]/

Numbers?

/[0-9]

We can use parenthesis `()` to group things together

Does the string contain any words with “ain” in them?

/(ain)/

There are many “helpers” that you can use to find more interesting parts of the string

\s = White space
\b = Word Boundary
\w = All word characters (A-Z,a-z,0-9)
^ = Beginning of the string
$ = End of the string
. = Any Character
* = Greedy (keep going!)

So, with combinations of helpers, ranges, and groups, you get pretty intricate

/s.?([A-Za-z]in)\s+/g

a lowercase s, followed by any letters, followed by “in” followed by a space … in other words “spain”, but not “rain”

JavaScript Methods and Practical Use

Boolean Checks (Does my string match the regex?)

  let str = "The rain in Spain falls mainly in the plain";
  // Are there any words ending with some letter before "in"
  let regex = /([A-Za-z]in)\b/g;
  regex.test(str);  // true

What is in my string? (can I get a list of all the matches?)

  let str = "The rain in Spain falls mainly in the plain";
  // The first letter in every word
  let regex = /(\b\w)/g;
  str.match(regex);  // T,r,i,S,f,m,i,t,p

Replace something in my string

  let str = "The rain in Spain falls mainly in the plain";
  // The first letter in every word
  let regex = /(\b\w)/g;
  str.replace(regex,'-');  // -he -ain -n -pain -alls -ainly -n -he -lain"

Replace something in my string, but use a function to do it

  // Camel Case a String ...

  let str = "The rain in Spain falls mainly in the plain";
  let regex = /\b(\w)/g;
  let newString = str.replace(regex, (rawMatch, matchedThing , idx) => {
      return idx === 0 ? matchedThing.toLowerCase() : matchedThing.toUpperCase();
    }).replace(/\W/g, '')

  console.log(newString); // theRainInSpainFallsMainlyInThePlain