Learn All the Regular Expression(Regex)/ Cheat Sheet You need for JavaScript in One Blog.

What is Regular Expression?

Regular Expressions

• Regex, or regular expressions, are patterns that help programmers match, search, and replace text.
• Regular expressions are extremely powerful, but they can be difficult to read since they use special characters to make more complex, flexible matches.

  What is .test( ) Method?

• Regexes can be used in JavaScript in multiple ways.
• Testing a regex can be done using the .test() method.

• The .test() method takes a regex, applies it to a string (which is enclosed in parentheses), and returns true or false based on whether your pattern found something.

• For Example:

  let testStr = "I Love India";
  let testRegex = /India/;
  testRegex.test(testStr);
  console.log(testRegex.test(testStr));
  //The test method here returns true. Because India is found in testStr.
  let testRegexx = /America/;
  testRegexx.test(testStr);
  console.log(testRegexx.test(testStr));
  //The test method here returns false.
  //Because America is not found in testStr.
  

  Match Literal Strings

• This method takes a regex, applies it to a string (which is enclosed in parentheses), and returns true or false based on whether your pattern found something.

• It is not Case Sensitive.

• Example:

let waldoIsHiding = "Somewhere Waldo is hiding in this text.";
let waldoRegex = /Waldo/; //returns true as waldoRegex value Waldo 
//matches waldoIsHiding
let result = waldoRegex.test(waldoIsHiding);
console.log(result);
//The Output is true

Match a Literal String with Different Possibilities

• Using regexes like /coding/, you can look for the pattern coding in another string.
• This is powerful to search single strings, but it's limited to only one pattern. You can search for multiple patterns using the alternation or OR operator: |.
• This operator matches patterns either before or after it. For example, if you wanted to match the strings yes or no, the regex you want is /yes|no/.

• You can also search for more than just two patterns. You can do this by adding more patterns with more OR operators separating them, like /yes|no|maybe/.

• For Example:

  let petString = "James has a pet cat.";
  let petRegex = /dog|cat|bird|fish/;
  //Checks the value of dog,cat, bird ,or fish in petString 
  //String returns true if either one of them is found else return false
  let result = petRegex.test(petString);
  //The Output is true
  

  Ignore Case While Matching

• Case (or sometimes letter case) is the difference between uppercase letters and lowercase letters.
• Examples of uppercase are A, B, and C.

• Examples of lowercase are a, b, and c.

• You can match both cases using what is called a flag.

• There are other flags but here you'll focus on the flag that ignores case - the i flag.
• You can use it by appending it to the regex. An example of using this flag is /ignorecase/i.

• This regex can match the strings ignorecase, igNoreCase, and IgnoreCase.

• For Example:

  let myString = "FreeCoding";
  let fccRegex = /freeCOdiNg/i; 
  // Ignore the value is upperCase or lowerCase
  let result = fccRegex.test(myString);
  console.log(result);
  //The output is true
  

  Extract Exact Matches

• You can extract the actual matches you found with the .match() method.
• To use the .match() method, apply the method on a string and pass in the regex inside the parentheses.

• For Example:

  let extractStr = "Extract the word 'coding' from this string.";
  let codingRegex = /coding/;
  let result = extractStr.match(codingRegex);
  //Return the Exact match with a value coding
  console.log(result);
  //The output is ["coding"]
  

  Find More Than the First Match

• To search or extract a pattern more than once,we use the global search flag: g.

• For Example:

• let testStr = "Repeat, Repeat, Repeat";
  let ourRegex = /Repeat/g;
  let result=testStr.match(ourRegex);
  console.log(result)
  //The output is ["Repeat", "Repeat", "Repeat"]
  

  Match Anything with Wildcard Period

• The wildcard character . will match any one character.

• The wildcard is also called dot and period.
• You can use the wildcard character just like any other character in the regex.

• For example, if you wanted to match hug, huh, hut, and hum, you can use the regex /hu./ to match all four words.

• For Example:

  let humStr = "I'll hum a song";
  let hugStr = "Bear hug";
  let huRegex = /hu./;
  huRegex.test(humStr);
  huRegex.test(hugStr);
  //Both of these test calls would return true.
  

  Match Letters of the Alphabet

• To match the Letters of the Alphabet in the string from a particular range

• Syntax:

  /[a-e]/ //this expression would match alphabets from a to e in LowerCase.
  /[A-Z]/ //this expression would match alphabets from A to Z in UpperCase.
  /[A-Z]|[a-z]/ //this expression would match both in uppercase and lowercase from a to z.
  

• For Example:

  let testStr = "Regexes can be used in JavaScript in multiple ways.Testing a regex can be done using the .test() method.";
  let upRegex =(/[A-Z]/g);//for upperCase
  let loRegex =( /[a-z]/g);//for upperCase
  let bothRegex = (/[A-Z]|[a-z]/g);//for upperCase
  let resultUp=testStr.match(upRegex);
  let resultLow=testStr.match(loRegex);
  let resultBoth=testStr.match(bothRegex);
  console.log("UpperCase found: "+resultUp.join(""))
  console.log("LowerCase found: "+resultLow.join(""))
  console.log("UpperCase & LowerCase found: "+resultBoth.join(""))
  

• Output:

• UpperCase found: RJST

• LowerCase found: egexescanbeusedinavacriptinmultiplewaysestingaregexcanbedoneusingthetestmethod

• UpperCase & LowerCase found: RegexescanbeusedinJavaScriptinmultiplew aysTestingaregexcanbedoneusingthetestmethod

Match Characters that Occur One or More Times

• You case + character to find the Characters that Occur One or More Times.

• For example, /a+/g would find one match in abc and return ["a"]. Because of the +, it would also find a single match in aabc and return ["aa"].

• let difficultSpelling = "Mississippi";
  let myRegex = /s+/gi; // Matches all s in the string
  let result = difficultSpelling.match(myRegex);
  console.log(result);
  //The output :["ss", "ss"]
  

  Find Characters with Lazy Matching

• In regular expressions, a greedy match finds the longest possible part of a string that fits the regex pattern and returns it as a match. The alternative is called a lazy match, which finds the smallest possible part of the string that satisfies the regex pattern.

• For Example, you can use the ? character to change it to lazy matching. "titanic" matched against the adjusted regex of /t[a-z]*?i/ returns ["ti"].

Match All Letters and Numbers

• Syntax: /[A-Za-z0-9_]+/

• For Example:

  let quoteSample = "The five boxing wizards jump quickly.";
  let alphabetRegexV2 = /\w/gi;//Syntax to match All Letters and Numbers
  let result = quoteSample.match(alphabetRegexV2).length;
  console.log(result)
  //The output is 31 because there are 21 Letters and Numbers in total
  

  Match All Numbers

• Syntax: /[0-9_]+/

• For Example:

  let quoteSample = "The five boxing 48649wizards jump quickl46498y.";
  let alphabetRegexV2 = /[0-9]/gi;//Syntax to match All Numbers
  let result = quoteSample.match(alphabetRegexV2);
  console.log(result)
  //Output : ["4", "8", "6", "4", "9", "4", "6", "4", "9", "8"]
  

  Match All Non Numbers

• Syntax: /[0-9_]+/D

• For Example:

  let quoteSample = "The five boxing 48649wizards jump quickl46498y.";
  let alphabetRegexV2 = /\D/g;//Syntax to match All Numbers
  let result = quoteSample.match(alphabetRegexV2).join("");
  console.log(result)
  //Output : The five boxing wizards jump quickly.
  

  Match Whitespace & Match Non Whitespace:

• Syntax:

  /\s/g //Matches All White Spaces and return them
  /\S/g //Matches All Non White Spaces and return them
  

• For Example

  let whiteSpace = "Whitespace. Whitespace everywhere!"
  let spaceRegex = /\s/g;
  let noSpaceRegex = /\S/g;
  let resultWhiteSpace=whiteSpace.match(spaceRegex);
  console.log("whiteSpace"+resultWhiteSpace) //Returns Whitespace Characters
  let resultNoWhiteSpace=whiteSpace.match(noSpaceRegex).join("");
  console.log("No Whitespace: "+resultNoWhiteSpace); //Returns Non Whitespace Characters
  

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Reference: I Have Simplified the things I learned from FreeCodeCamp.

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