Regular Expressions Cheat Sheet in Node.js

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1. Introduction

Regular expressions are a topic that confuses and struggles a lot of developers due to its crypt syntax.

A regular expression is a string that contains a search pattern (ex. **[0–9]{2}\ **this pattern accepts a 2 digit number), and we use it to find those patterns into texts.

2. How to Create a RegExp

In javascript we can create RegExp with 2 ways:

// a string with a format /<RegExp>/;
**var twoDigitRegExp = /[0–9]{2}/;**

// Construct a new RegExp object
**var twoDigitRegExp = new RegExp(‘[0–9]{2}’);**

3. RegExp Building Blocks

Quantifiers

**p*** --> The ***** will match the previous **p** char for 0 or more times.
Example **/js*/** will accept j (s char has **zero** times) , js (s char has **one** time) , jss (s char has **two** times).

**p+** --> The **+** will match the previous **p** char for 1 or more times.
Example **/js+/** will not  accept j (s char has **zero** times) but will accept js (s char has **one** time) , jss (s char has **two** times).

**p?** --> The **?** will match the previous  **p** char for 0 or one time.
Example **/js?/** will accept j (s char has **zero** times), js (s char has **one** time) but not accept jss (s char has **two** times).

**p{n}** --> The **{n}** will match the previous  **p** char for n times.
Example **/js{1}/** will accept js (s char has **one** time) but not accept jss (s char has **two** times).

**p{n,}** --> The **{n}** will match the previous  **p** char for at least n times.

**p{x,y}** --> The **{x,y}** will match the previous  **p** char from x to y times.

Character classes

**/.p/ **-->** **The **.** will match every charatecter before **p. **Example **/.p/ **will accept **keep , deep **but not accept **js.**

**/\d/ **-->** **The **\d** will match every number from [0-9]. Example **/festival\d/** will accept festival2020 , festival 2019 but not festivalBeer.

**/\D/ **-->** **The **\D** will match every number is **not** from [0-9]. Example **/festival\D/** will accept festivalBeer but not festival2020.

**/\w/ **-->** **The **\w** will match every character **in** latin alphabet include underscore. Example **/festival\w/** will accept festival2020 , festival 2019 but not festival$$.

**/\W/ **-->** **The **\W** will match every character **not** **in** latin alphabet include underscore. Example **/festival\W/** will accept festival#$ but not accept festival2019.

**/\s/ **-->** **The **\s** will match a single white space character, including space, tab, form feed, line feed, and other Unicode spaces. Example **/festival \s2020/** will match ' 2020' at 'festival 2020'.

**/\S/ **-->** **The **\S** will matche a single character other than white space. Example **/\S\w*/** will match 'festival' at 'festival 2020'.

Groups and ranges

**/gin|wine/ **-->** **The **| **is the or operator**. **Example **/gin|wine/ **will match 'gin' or 'wine' at 'Gin is my favorite drink'.

**/[A-Z]/ **-->** **The **[] **indicates a characters set** **to match every char in the set. Examples of character sets [A-Z] matches any char from A-Z, [@#] matches only @ and # and so on**. **Example **/[@#]\w*/g **will match '@wine' and '#scotch' at '@wine $gin #scotch'.

**/[^A-Z]/ **-->** **The **^ **indicates the **not in operator **a characters set.

**/(@|#)/ --> The () **indicates capturing group. Example **/(@|#)(gin|wine)/g ** at '**@wine @gin scotch' **will match '@wine' and '@gin'

Regular expression flags

**/(@|#)(gin|wine)/g **--> Global search with **g flag** the regExp **/(@|#)(gin|wine)/g **will check every word that matches. Example **/(@|#)(gin|wine)/g **at '@wine @gin scotch' will match '@wine' and '@gin' but without **g flag **will match only first word** **'@wine'

**/(@|#)(gin|wine)/i **-->** **Case sensitive search with **i flag**. Example **/(@|#)(gin|wine)/i **at '@WINE @gin scotch' will match '@WINE'.

**/(@|#)(gin|wine)/gi --> **Combine **g and i flags** will check all words and with case sensitive. Example **/(@|#)(gin|wine)/i **at '@WINE @gin scotch' will match '@WINE' and '@gin'.

RegExp Assertions

**@(?=gin)** --> **Lookahead assertion: **Matches '@' only if '@' **is **followed by 'gin'.

**@(?!gin)** --> **Lookahead negative assertion: **Matches '@' only if '@' **is not** followed by 'gin'.

**(?<=gin)@** --> **Lookbehind assertion: **Matches '@' only if 'gin' **is **followed by '@'.

**(?<!gin)@** --> **Lookbehind negative assertion: **Matches '@' only if 'gin' **is not **followed by '@'.

Build a Flush Message Middleware with Node.js Learn how to build a flush messages middleware system with node.js and express.js from scratchmedium.com

4. Real Examples

Regular expressions are used mainly in two topics:

1. Input validation

2. Search-Replace patterns in texts

Input Validation Examples

Validate an email with the following format:

1. Name with at least 4 digits

2. Name without special characters ^ < > ( ) [ ] \\ / . , ; : \s @ ’

3. E-mail must have @

4. Domain name with at least 4 digits

5. Domain name without special characters ^ < > ( ) [ ] \\ / . , ; : \s @ ’

6. Domain extension only .com or .net

   var regExpEmail = /([A-Z]|[a-z]|[^<>()[]\/.,;:\s@"]){4,}@([A-Z]|[a-z]|[^<>()[]\/.,;:\s@"]){4,}.(com|net)/;

   var email1 = 'petranb2@gmail.com'

   var email2 = 'petran@pkcoding.net'

   var email3 = 'petran@pkcoding.org'

   var email4 = 'pe<>ran@pkcoding.org'

   console.log(Test ${email1}:+regExpEmail.test(email1));

   console.log(Test ${email2}:+regExpEmail.test(email2));

   console.log(Test ${email3}:+regExpEmail.test(email3));

   console.log(Test ${email4}:+regExpEmail.test(email4));

   <-- Console output --> Test petranb2@gmail.com:true Test petran@pkcoding.net:true Test petran@pkcoding.org:false Test pe<>ran@pkcoding.org:false

Validate a password with the following format:

1. Password at least 6 digits.

2. At least one lowercase

3. At least one uppercase

4. At least one special character from @ # $ % ^ & *

   var regExpPassword = /(?=.[a-z])(?=.[A-Z])(?=.[0-9])(?=.[!@#$%^&*])(?=.{6,})/;

   var password1 = 'J@v@scr1pt'

   var password2 = 'N0d3js'

   var password3 = 'umbr3ll@'

   var password4 = 'A1rpl@ne'

   console.log(Test ${password1}:+regExpPassword.test(password1));

   console.log(Test ${password2}:+regExpPassword.test(password2));

   console.log(Test ${password3}:+regExpPassword.test(password3));

   console.log(Test ${password4}:+regExpPassword.test(password4));

   **<-- Console output --> **Test J@v@scr1pt:true Test N0d3js:false Test umbr3ll@:false Test A1rpl@ne:true

Validate a product code with the following format:

1. Code must have format (XXX)-XXX

2. Each X must be a number

   var regExpProductCode = /((\d{3}))-(\d{3})/;

   var product1 = '(854)-458'

   var product2 = '(1234)-4sw'

   var product3 = '256-789'

   var product4 = '(123)-456'

   console.log(Test ${product1}:+regExpProductCode.test(product1));

   console.log(Test ${product2}:+regExpProductCode.test(product2));

   console.log(Test ${product3}:+regExpProductCode.test(product3));

   console.log(Test ${product4}:+regExpProductCode.test(product4));

   **<-- Console output --> **Test (854)-458:true Test (1234)-4sw:false Test 256-789:false Test (123)-456:true

Search-Replace patterns in texts

Search if after a dot the text has 2 or more spaces and replace it with 1 space.

var regExpText = **/. {2,}(?=[A-Z|a-z])/**;

var text = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.   Nunc';

console.log(`Test ${text}:<<`+regExpText.exec(text)+`>>`);

console.log(text.replace(regExpText,'. '));

**<-- Console output -->**

Test Lorem ipsum dolor sit amet, consectetur adipiscing elit.   Nunc:<<.   >>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc

Search if the text begins with a lowercase after a dot and replace it with uppercase

var regExpText = **/(?<=\.\s{1,})[a-z]/**;

var text = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. nunc';

console.log(`Test ${text}:<<`+regExpText.exec(text)+`>>`);

var lowerCaseChar = regExpText.exec(text)[0];

console.log(text.replace(regExpText,lowerCaseChar.toUpperCase()));

**<-- Console output -->
**Test Lorem ipsum dolor sit amet, consectetur adipiscing elit. nunc:<<n>>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc

5. Security Tip

Regular Expressions can be a great alternative, but a badly written Regex could be CPU greedy and block the node.js event loop.

Read about how to prevent these circumstances at: How a RegEx can bring your Node.js service down *The use of Regular Expressions (RegEx) is quite common among software engineers and DevOps or IT roles where they…*medium.com

6.Conclusion

Learning and using regular expressions can elevate you to a great software developer.

References:

1. MDN web docs

2. Wiki

Special Thanks to Liran Tal for the security advice

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