Use case of RegEx in JavaScript

Published on: 16th Mar 2023

Updated on: 18th Mar 2023

Overview

String operation is very common in any program especially those requires in processing user input. For example, in Membership Management System and the back end system that is processing the JSON string from other system or user.

Use case

Ensure that the first character is in upper case,

let user_input = 'abc';
if (!/^[A-Z]/.test(user_input)) {
    console.log('The user input is not start with upper case');
}

Using RegEx like SQL IN operator,
```
let user_input = 'BANANA';
if (/APPLE|BANANA/.test(user_input)) {
    console.log('The user input has either apple or banana');
}
```
In case the user might key in the value in lower or mixed case, add i flag (case insensitive) after the second '/',
```
let user_input = 'Banana';
if (/APPLE|BANANA/i.test(user_input)) {
    console.log('The user input has either apple or banana');
}
```
This useful technique is good for validating the city name in the address fields.

Unsuitable use case: for example, the user input is limited to either 1, 3 or 5. By using following RegEx, it returns true which is an unexpected result.
```
console.log(/1|3|5/i.test('13'));
// output: true
```
The correct way to get the expected result is to use indexOf() function provided by an array,
```
console.log(['1', '3', '5'].indexOf('13') >= 0);
// output: false
```

Using RegEx like SQL LIKE operator,

console.log(/m.*ing/.test('good morning'));
// output: true

console.log(/m.*ing/.test('good mornin'));
// output: false

console.log(/m.*ing/.test('good afternoon'));
// output: false

Extract a text block from a very long string,
```
let some_string = '{"name":"mike"}{"name":"john"}';

const RG = /{.*?}/g;

// find the first block
let match = RG.exec(some_string);
console.log(match[1]);
// output: {"name":"mike"}

// search for next block
match = RG.exec(some_string);
console.log(match[1]);
// output: {"name":"john"}
```
Where
- g indicates global (i.e., continue look for next matches from the last position).
- .* means any character.
- ? means the searching process will stop once it found the first '}' symbol. Without this, the result might return the entire string.
In case you want to use RG again for other value in some_string, you must reset the last search position.
```
RG.lastIndex = -1;
```
Extract multiple text blocks with capturing group (the ( and ) that wraps the \d) from the user input,
```
let user_input = '1999-02-03';
let match = /(\d{4})-(\d{2})-(\d{2})/.exec(user_input);

// output: match[1]=1999
// output: match[2]=02
// output: match[3]=03
```
where
- \d indicates that we are looking for "a single digit" or "numeric".
- {4} means 4 consecutive digits.
- The ( and ) is the capturing group and the matches can be accessed with match[1], match[2] and match[3] in this example.

Replaces all occurrences of carrige and line feed characters in the given text,

let user_input = 'Helo Mike,\r\nWelcome to our blog\r\nSome more information here..';
let user_input2 = user_input.replace(/\r\n/g, '<br/>');

Conclusion

Using RegEx to validate and process user input makes the program simpler and easier to maintain.

For a more complex usage of RegEx, please refers to one of the earlier post: Parsing CREATE TABLE statement with Regex in Node.js

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Author

Lau Hon Wan, software developer.

Use case of RegEx in JavaScript

Overview

Use case

Conclusion

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