RegEx (Regular Expressions)

Q. What is RegEx? A sequence of characters that define a pattern

Q. What is RegEx used for? Validation — search, match, and replace text in strings

Q. How do you implement RegEx in JavaScript?

a) Regular Expression Literal

• The forward slashes delimit the regex

const regExExample = /hello?/;

b) RegExp( ) Constructor function

const text= "hello?";
const re = new RegExp(text);

1. Literal Characters

• Purpose: matches the exact characters we want
• Ex. pattern=”Fred” → Fred ✅ , fred ❎

2. Character Class: [ ] , ( )

• Purpose: matches the characters in the brackets

• Ex. pattern=”[fF]red” → Fred ✅ , fred ✅ , fFred ❎
• Ex. pattern=”[5–9]” → matches a single digit from 5 to 9
• Ex. pattern=”(hi!){2}” → hi!hi! ✅

3. Negated Character Class: [ ^]

• Purpose: matches any character EXCEPT the character in the bracket
• Ex. pattern=”p[^ua]” → pi ✅ , po ✅ , pu ❎ , pa ❎

Matches p followed by any single letter except u or a

4. Anchors: ^ , $

• Purpose: matches the string that only has the character(s) we want

NOTE: It accounts for the character(s) and the position

• Ex. pattern=”^dog” → dog hi ✅ , hi dog ❎ , doggy hi ✅
• Ex. pattern=”dog$” → dog hi ❎ , hi dog ✅
• Ex. pattern=”^dog$” → dog ✅ , dog hi ❎ , hi dog ❎

5. Shorthand Character Class: \d, \w, \s, \D, \W, \S, .

• Purpose: matches the characters/digits we want

6. Quantifiers: { } ,* ,+ ,?

• Purpose: concise — no need to repeat same character class when matching 1+

i) { }

• Ex. pattern=”\d{3}” → matches 3 digits
• Ex. pattern=”[A-Z]{1, 5}” → matches between 1 & 5 capital letters
• Ex. pattern=”\d{5, } → matches 5 to an infinite number of digits

ii) *

• Ex. pattern=”ab\d*” → ab ✅ , ab1 ✅ , ab1234 ✅

NOTE: Can be replaced with “ab\d{0, }”

iii) +

• Ex. pattern=”ab\d+” → ab1 ✅ , ab1234 ✅ , ab ❎ , ab1234d ❎

iv) ?

• Ex. pattern=”ab\d?” → ab ✅ , ab1 ✅ , ab1234 ❎

7. Escape Special Characters: \

• Purpose: to include special characters in regex → precede with \ (backslash)

NOTE: No need to escape special character within character class ([ ])

• Ex. pattern=”\{\d,\d}\” → {3,5} ✅ 3,5 ❎

8. Alternation: |

• Purpose: search for one of several characters of words (similar to JS’ ||)
• Ex. pattern=”I have a (dog|cat|bird|fish)./” → I have a dog. ✅ , I have a cat. ✅ , I have a bird. ✅ , I have a fish. ✅

9. Boundary: \b

• Purpose: Search for whole words only
• Ex. pattern=”\bDog\b” → Dog ✅ , Doggy ❎ , My Dog is Sparky ✅

Resources

1. Here’s a concise 2-page RegEx cheatsheet I made:

https://docs.google.com/document/d/13Q5Yx_I1YscShFJTqhgA6s4nUnMXc5lVlY0JlcxRu-4/edit?usp=sharing

2. https://regex101.com/

3. https://regexr.com/