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Network & Web · In-Browser Regular Expression Lab

Localhost Regex — Match Localhost and 127.0.0.1 Ports

Matches local development origins including http://localhost:PORT and 127.0.0.1 with ports 1024-65535.

Flags:
//none
Match Found
Load Sample:
Match Inspection & Details1 match
Match #1:http://localhost:3000

Test Samples Verification Matrix

Expected Valid Samples (Should Match)

http://localhost:3000 Pass
http://127.0.0.1:8080 Pass
https://localhost:443 Pass

Expected Invalid Samples (Must Reject)

http://castov.com Rejected (Pass)
ftp://localhost Rejected (Pass)
http://localhost:999999 Rejected (Pass)
Code Implementation Generator
// JavaScript / TypeScript (ES6+)
const pattern = /^https?:\/\/(?:localhost|127\.0\.0\.1)(?::[0-9]{1,5})?$/;
const text = "http://localhost:3000";

const isMatch = pattern.test(text);
console.log("Is Match:", isMatch);

if (isMatch) {
  const matches = text.match(pattern);
  console.log("Matches:", matches);
}

Regular Expression Token Breakdown

Matches local dev server origins for CORS and redirect URI whitelists. Each component of the regular expression serves a distinct syntactical purpose as mapped below:

Token PatternName / ComponentFunction & Rule
^https?:\/\/Protocolhttp:// or https://.
(?:localhost|127\.0\.0\.1)HostMatches localhost or 127.0.0.1.
(?::[0-9]{1,5})?$PortOptional port number up to 65535.

Frequently Asked Questions

Why use this in CORS middleware?
Allows local development servers to call APIs without compromising production origins.

Related Regular Expressions

Browse All 40 Patterns
Network & Web

HTTP & HTTPS URL Validation

Matches fully qualified URLs starting with http:// or https://, including subdomains, paths, ports, and query parameters.

/^https?:\/\/(?:www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}(?:\.[a-zA-Z0-9()]{1,6})?(?::[0-9]{1,5})?\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*)$/i
Test regex
Network & Web

Domain Name Matcher

Validates Fully Qualified Domain Names (FQDN) adhering to DNS hostname specification (RFC 1035).

/^(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,}$/
Test regex
Network & Web

IPv4 Address Validator

Strictly validates IPv4 addresses ensuring each octet is within the valid 0 to 255 numerical range.

/^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$/
Test regex