Regex Expressions in JS
JavaScript is a programming language that supports regular expressions (regex). A regular expression is a sequence of characters that specifies a search pattern. With regex, we can search and manipulate text data in a more efficient and flexible way. In this document, we will explore how to use regular expressions in JavaScript.
Flags:
g: Global match. This flag indicates that the regular expression should match all occurrences in the input string, rather than just the first one.i: Case-insensitive match. This flag indicates that the regular expression should match regardless of the casing of the input string.m: Multiline match. This flag indicates that the regular expression should match across multiple lines of the input string.
Patterns:
- Matching any character:
. - Matching any digit:
\\d - Matching any non-digit:
\\D - Matching any whitespace character:
\\s - Matching any non-whitespace character:
\\S - Matching any word character:
\\w - Matching any non-word character:
\\W - Matching a specific number of characters:
{n} - Matching zero or more occurrences: ``
- Matching one or more occurrences:
+ - Matching zero or one occurrence:
? - Matching the start of a string:
^ - Matching the end of a string:
$ - Matching a range of characters:
[abc] - Matching any character not in a range:
[^abc] - Matching any character in a range of characters:
[a-z] - Matching any character not in a range of characters:
[^a-z] - Matching any character except newline:
[\\s\\S] - Matching a specific character:
\\ - Matching a specific word boundary:
\\b - Matching a negative word boundary:
\\B - Capturing a group of characters:
( ) - Non-capturing a group of characters:
(?: ) - Matching one of several patterns:
(pattern1|pattern2|pattern3) - Using a backreference to match a previous captured group:
\\n - Matching any number of characters lazily:
? - Matching one or more characters lazily:
+? - Matching a specific number of characters lazily:
{n}? - Using lookahead to match a pattern only if it’s followed by another pattern:
(?= ) - Using lookbehind to match a pattern only if it’s preceded by another pattern:
(?<= )
// Matches all occurrences of the word "dog"
const regex = /dog/g;
// Matches the word "hello" regardless of case
const regex = /hello/i;
// Matches the pattern "cat" at the beginning of a string
const regex = /^cat/;
// Matches the pattern "bat" at the end of a string
const regex = /bat$/;
// Matches any vowel character
const regex = /[aeiou]/;
// Matches any non-vowel character
const regex = /[^aeiou]/;
// Matches either "cat" or "dog"
const regex = /cat|dog/;
// Groups together "hello" and "world" and captures the result
const regex = /(hello) (world)/;Most Used Regex Patterns
- Email addresses:
\\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Z|a-z]{2,}\\b- this matches most standard email addresses and ensures that the domain name ends with at least two letters.
2. Phone numbers:
\\b\\d{3}[-.]?\\d{3}[-.]?\\d{4}\\b- this matches most US phone numbers, with or without dashes or periods between groups of digits.
3. URLs:
(http(s)?://)?([\\w-]+\\.)+[\\w-]+(/[\\w- ;,./?%&=]*)?- this matches most standard URLs, including those with or without thehttp://orhttps://prefixes, and with or without query parameters.
4. Postal codes:
[A-Z]\\d[A-Z] ?\\d[A-Z]\\d- this matches most Canadian postal codes, which have the format A1A 1A1, with the first three characters being a letter, followed by a digit, followed by another letter, and then a space, and then a digit, followed by another letter, and then a digit.
5. IP addresses:
\\b\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\b- this matches most standard IP addresses, with each group of digits separated by periods.
6. Dates:
\\b\\d{1,2}[\\/\\-]\\d{1,2}[\\/\\-]\\d{4}\\b- this matches most standard date formats, with the month and day separated by either a slash or a dash, and the year consisting of four digits.
7. Credit card numbers:
\\b\\d{4}([ \\-]?\\d{4}){3}\\b- this matches most standard credit card numbers, with groups of four digits separated by either spaces or dashes.
8. Social Security numbers:
\\b\\d{3}[ \\-]?\\d{2}[ \\-]?\\d{4}\\b- this matches most standard SSN formats, with groups of three, two, and four digits separated by either spaces or dashes.
9. File paths:
^[A-Za-z]:[\\\\/].*- this matches most standard file paths on Windows systems, with the drive letter followed by a colon, and then the rest of the path separated by either backslashes or forward slashes.
Creating a Regex Object
In JavaScript, we can create a regex object using the RegExp constructor or by using a regex literal. Here is an example:
// Using RegExp constructor
let regex1 = new RegExp('hello');
console.log(regex1.test('hello world')); // true
// Using regex literal
let regex2 = /hello/;
console.log(regex2.test('world hello')); // trueMatching a Pattern
To match a pattern in a string, we can use the match method. This method returns an array of matches or null if no matches are found. Here is an example:
let str = 'The quick brown fox jumps over the lazy dog';
let regex = /the/gi;
let matches = str.match(regex);
console.log(matches); // ["The", "the"]In this example, we used the g and i flags to match all occurrences of the word "the" in a case-insensitive way.
Replacing a Pattern
To replace a pattern in a string, we can use the replace method. This method returns a new string with the replaced value. Here is an example:
let str = 'JavaScript is a popular programming language';
let regex = /JavaScript/;
let newStr = str.replace(regex, 'JS');
console.log(newStr); // "JS is a popular programming language"In this example, we used the replace method to replace the word "JavaScript" with "JS" in the string.
Conclusion
Regular expressions are a powerful tool in JavaScript for searching and manipulating text data. We can create regex objects using the RegExp constructor or regex literals. We can match a pattern using the match method and replace a pattern using the replace method. With regular expressions, we can write more efficient and flexible code.