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RegExpIs a regular expression used to match a fixed set of characters in a target string using TypeScript.

RegExpObjects have many properties and methods. Each property has different options when pattern matching.

Using regular expressions in TypeScript

Regex can be implemented in TypeScript in two ways. One of them is to assign a regular expression literal type RegExp.

const rExp : RegExp = /[A-C]/g;

Another way to express regular expressions is through RegExpthe constructor.

const rExp : RegExp = new RegExp("[A-C]", "g");

For the rest of this article, you will use constant regular expressions to match test strings.

Use the method in TypeScript testto check if a pattern exists in the target string

RegExpThe test method can be used booleanto check whether a specific pattern or regular expression exists in the target string by returning a variable.

const targetString : string = "All is well";
// regex to check if 'All' word is present or not.
const rExp : RegExp = /All/;
console.log(rExp.test(targetString));

Output:

true

In TypeScript, use RegExpthe string matchor execmethod to find matches in a target string.

matchThe method on a string RegExpor execthe method on a string finds matches that correspond to a regular expression in the target string.

Both methods behave similarly in finding the first match in the target string.

const targetString : string = "All is well";
const rExp : RegExp = /All/;
console.log(rExp.exec(targetString));
console.log(targetString.match(rExp))

Output:

["All"]
["All"]

The functions differ in whether they find all occurrences of a matching string or search with a global flag set.

const targetString : string = "All is well and Beautiful";
const rExp : RegExp = /[A-C]/g;
console.log(rExp.exec(targetString));
console.log(targetString.match(rExp))

Output:

["A"]
["A", "B"]

However, execthe function can also find the total number of matches in the target string corresponding to the regular expression.

function getMatches( target : string, rExp : RegExp, matches : Array<RegExpExecArray> = []) {
    const matchIfAny  = rExp.exec(target);
    matchIfAny && matches.push(matchIfAny) && getMatches(target, rExp, matches);
    return matches;
}

const rExp : RegExp = /[A-C]/g

const test_string : string = "All is Well and Beautiful";

console.log(getMatches(test_string, rExp, []).map( arr => arr[0]));

Output:

["A", "B"]

RegExpUsing flags in literal expressions in TypeScript

Some common flags in regular expressions include the ignore case flag i, the global flag g, and the multiline flag m.

The following code snippet demonstrates how to use the iand mflags.

const targetString : string = `#1 First line
#2 Second Line
#3 Third liNe`;
// regex to check  word is present or not.
const rExp1 : RegExp = /line/g;
const rExp2 : RegExp = /line/gi;
const rExp3 : RegExp = /^#\d/gi;
const rExp4 : RegExp = /^#\d/gim;
console.log(targetString.match(rExp1));
console.log(targetString.match(rExp2));
console.log(targetString.match(rExp3));
console.log(targetString.match(rExp4));

Output:

["line"]
["line", "Line", "liNe"]
["#1"]
["#1", "#2", "#3"]

Thus, ithe flag captures all occurrences line, regardless of case.

mFlags are often useful for finding patterns at the beginning and end of a line.

In this example, mthe flag is set to find all matches that begin with a digit at the beginning of a line.