JavaScript RegExp – global

In JavaScript, a regular expression is a sequence of characters that define a search pattern. RegEx helps to find, replace, or validate strings, making it an essential tool in web development. One of the key features of RegEx is the global flag, denoted by the letter “g.” In this article, we will explore the global flag in JavaScript regular expressions.

The Global flag in Regular Expressions

The global flag is used in a regular expression to search for a pattern throughout the entire string. It ensures that the regular expression scans the entire string and returns all matches. Without the global flag, the RegEx would stop scanning as soon as it finds the first match.

Let’s consider an example: suppose we have a string, The quick brown fox jumps over the lazy dog., and we want to find all occurrences of the character “o.” Without the global flag, we would write the regular expression as follows:

const str = "The quick brown fox jumps over the lazy dog.";
const regex = /o/; //Regular expression without global flag
const output = str.match(regex); //["o"]

Here, “o” is the first match that the regular expression finds. However, with the global flag, the regular expression would find all occurrences of “o.”

const str = "The quick brown fox jumps over the lazy dog.";
const regex = /o/g; //Regular expression with global flag
const output = str.match(regex); //["o","o","o"]

Here, the regular expression regex finds all the occurrences of the character “o”. The match() function returns an array of all the matches.

We can use the global flag on any regular expression pattern, including groups, character sets, and quantifiers.

Global flag with RegExp Constructor

In addition to the forwardslash notation, we can create regular expression patterns with the RegExp() constructor. When using the constructor, we can pass the global flag as a second argument.

For instance, let’s consider the following regular expression:

const regex = /o/g;

We can create the same regular expression pattern using the RegExp() constructor as follows:

const regex = new RegExp("o", "g");

Here, the first argument is the pattern to match, and the second argument is the global flag.

We can also use variables with the constructor. For instance:

const char = "o";
const regex = new RegExp(char, "g");

Here, the variable char holds the pattern “o”.

Using the Global flag with the Replace() function

In JavaScript, we can use the replace() function to replace parts of a string that match a regular expression pattern. The replace() function takes two arguments: the pattern to search for and the replacement string.

When a regular expression pattern has the global flag, we can replace all instances of the pattern using the replace() function.

Let’s consider an example:

const str = "My name is John, John is my name.";
const regex = /John/g;
const output = str.replace(regex, "Peter");

Here, the regular expression regex matches all occurrences of “John”(["John", "John"]) and replaces them with “Peter.”

console.log(output); // My name is Peter, Peter is my name.

Global Flag with the Test() Function

The test() function is a regular expression method that returns a boolean value indicating whether a string matches a pattern. This function is useful when we want to check if a string contains a particular pattern.

When a regular expression pattern has the global flag, we can use the test() function to check if the whole string matches the pattern.

Let’s consider an example:

const str = "The quick brown fox jumps over the lazy dog.";
const regex = /cat/g;
const output = regex.test(str);

Here, cat is not present in the original string str, yet the regular expression with the global flag g returns true, indicating the presence of the pattern.

console.log(output); // false

Conclusion

The global flag g is an essential feature of RegEx in JavaScript. It helps to match all occurrences of a pattern within a string. The global flag can be used with functions like match(), replace(), and test() and used with the RegEx constructor. With global flag, we can efficiently manipulate strings and create powerful search patterns.