Flags and the JavaScript regex engine
Flags change how the regex engine behaves. g (global) finds every match in the text, not just the first — without it, .exec() only ever returns the first result. i (case-insensitive) ignores letter case. With m (multiline), ^ and $ match the start/end of each line instead of the whole string. With s (dotAll), the . character also matches line-break characters (\n), which it normally excludes.
JavaScript's built-in RegExp engine (the ECMAScript standard) doesn't support some advanced features found in PCRE (PHP, Perl) or Python's re module — such as recursive patterns or conditional expressions. Lookbehind ((?<=...) / (?<!...)) is now supported in modern browsers, but if your pattern also needs to run server-side in a different language or an older environment, it's worth keeping this portability difference in mind.
In practice, regex is commonly used to validate input formats like emails, phone numbers or postal codes, to extract structured data such as IP addresses or timestamps from log files, and for bulk search-and-replace operations in code editors.
When should you use Regex Tester?
The KEYDAL Regex Tester tool is a browser-based utility that developers, system administrators, SEO specialists and enterprise technology teams use in their daily operations. It requires no installation, is free, and produces results instantly. It is designed so local teams can run audits without connecting to server environments and run analyses without touching production.
Typical scenarios include: post-migration verification, comparing domain or hosting providers, diagnosing customer issues, security auditing (pre-pentest reconnaissance), root-cause analysis of email deliverability problems, validating CDN or proxy configuration, surfacing technical audit data for SEO teams, and rapid information gathering during incident response. You can copy results as text and share them or paste them into internal documentation.
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KEYDAL tools run stateless: domain names, IPs, URLs or other inputs are not persisted to any database. Logs are kept only for security purposes (rate limiting, abuse detection) and deleted within 30 days. For tools that handle sensitive data (tokens, API keys, JWTs), processing happens entirely in your browser — nothing is sent over the network. See our Privacy Policy for details.
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