Input boundary assertion: ^, $

An input boundary assertion checks if the current position in the string is an input boundary. An input boundary is the start or end of the string; or, if the m flag is set, the start or end of a line.




^ asserts that the current position is the start of input. $ asserts that the current position is the end of input. Both are assertions, so they don't consume any characters.

More precisely, ^ asserts that the character to the left is out of bounds of the string; $ asserts that the character to the right is out of bounds of the string. If the m flag is set, ^ also matches if the character to the left is a line terminator character, and $ also matches if the character to the right is a line terminator.

Unless the m flag is set, the ^ and $ assertions only make sense when placed at the boundaries of the pattern, because any other characters to the left or right of them would necessarily cause the assertion to fail.

The y flag doesn't change the meaning of these assertions — see also anchored sticky flag.


Removing trailing slashes

The following example removes trailing slashes from a URL string:

function removeTrailingSlash(url) {
  return url.replace(/\/$/, "");

removeTrailingSlash(""); // ""
removeTrailingSlash(""); // ""

Matching file extensions

The following example checks file types by matching the file extension, which always comes at the end of the string:

function isImage(filename) {
  return /\.(?:png|jpe?g|webp|avif|gif)$/i.test(filename);

isImage("image.png"); // true
isImage("image.jpg"); // true
isImage("image.pdf"); // false

Matching entire input

Sometimes you want to make sure that your regex matches the entire input, not just a substring of the input. For example, if you are determining if a string is a valid identifier, you can add input boundary assertions to both ends of the pattern:

function isValidIdentifier(str) {
  return /^[$_\p{ID_Start}][$_\p{ID_Continue}]*$/u.test(str);

isValidIdentifier("foo"); // true
isValidIdentifier("$1"); // true
isValidIdentifier("1foo"); // false
isValidIdentifier("  foo  "); // false

This function is useful when doing codegen (generating code using code), because you can use valid identifiers differently from other string properties, such as dot notation instead of bracket notation:

const variables = ["foo", "foo:bar", "  foo  "];

function toAssignment(key) {
  if (isValidIdentifier(key)) {
    return `globalThis.${key} = undefined;`;
  // JSON.stringify() escapes quotes and other special characters
  return `globalThis[${JSON.stringify(key)}] = undefined;`;

const statements ="\n");

// = undefined;
// globalThis["foo:bar"] = undefined;
// globalThis["  foo  "] = undefined;


ECMAScript Language Specification
# prod-Assertion

Browser compatibility

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See also