- Enter your regular expression pattern in the top input field.
- Select the desired flags (e.g., 'g' for global search, 'i' for case-insensitive).
- Type or paste your test text in the 'Test String' area.
- View the highlighted matches instantly in the 'Match Result' box.
- Optionally, use the 'Substitution' section to test replacement logic.
What is a regular expression (regex)?
A regular expression (regex) is a sequence of characters that defines a search pattern for text matching. It is commonly used in programming for tasks like validation, searching, and text manipulation. Regex patterns can match specific strings, character ranges, or complex text structures.
Is Regex Tester free to use?
Yes, Regex Tester is completely free to use with no registration required. All features including pattern testing, match highlighting, and substitution preview are available at no cost. You can use it unlimited times for all your regex testing needs.
Is my data safe with Regex Tester?
Yes, your data is completely safe. All regex processing happens locally in your browser, and no data is sent to any server. Your test strings and patterns never leave your device, ensuring complete privacy and security.
How to test a regex pattern?
Enter your regex pattern in the input field, select appropriate flags, and type your test text. Matches are highlighted instantly in real-time. You can also test replacement patterns using the substitution feature to see how your regex transforms text.
Which regex engine is used?
This tool uses the native JavaScript RegExp engine built into your browser. It supports standard regex syntax including character classes, quantifiers, groups, and lookahead/lookbehind assertions available in modern JavaScript.
What does the 'g' flag do?
The 'g' (global) flag finds all matches in the text instead of stopping at the first match. Without this flag, only the first occurrence is matched. Use it when you need to find or replace multiple occurrences of a pattern.
Regular Expressions Complete Guide [2026] - From Beginner to Expert
Master regular expression syntax, principles, and practical applications. Deep dive into common regex patterns (email, phone, URL, etc.) with multi-language code examples to become a regex expert.
Context Window and Token Complete Guide: LLM Tokenization, Counting Methods, and Cost Optimization
Deep dive into Token and Context Window concepts in large language models, including BPE, WordPiece tokenization algorithms, model context window comparison, and practical methods for token counting and cost optimization.
NLP Natural Language Processing Complete Guide: From Tokenization to Large Language Models
A comprehensive guide to NLP natural language processing, covering tokenization, named entity recognition, sentiment analysis, machine translation, and mainstream models like BERT and GPT.