Regex Tester & Parser Playground

Test regular expressions in real time with flags, match index details, and parsed group output.

PatternFlags

Test Text

Matches found: 2

Match List

Index: 14
Match: help@example.com
Groups: None
Index: 34
Match: admin@codecompileronline.com
Groups: None

Parsed JSON Output

[
  {
    "index": 14,
    "value": "help@example.com",
    "groups": []
  },
  {
    "index": 34,
    "value": "admin@codecompileronline.com",
    "groups": []
  }
]

How to use regex tester

Enter pattern and flags, then paste text input. Review match index and group details to debug regular expression behavior quickly.

Common regex use cases

Validate email-like strings, extract IDs from logs, parse URLs, and test capture groups before shipping production code.

Common Regex Errors

Invalid regular expression

Check unmatched parentheses, brackets, or escape sequences.

No matches found

Verify flags like g, i, or m and confirm your pattern fits input text.

Unexpected capture groups

Use non-capturing groups (?:...) where you do not need group outputs.

Basic Regex Topics with Examples

Learn character classes, quantifiers, and anchors before writing advanced production patterns.

Character Classes

Character classes let you match groups like digits, words, and whitespace. They are the first step in practical regex building.

\d+        // one or more digits
\w+        // one or more word characters
[a-zA-Z]+  // only letters

Key takeaway: Use character classes to narrow matches before adding complexity.

Quantifiers

Quantifiers define how many times a token appears. They make patterns flexible for varying input lengths.

a*   // zero or more
a+   // one or more
a?   // optional
a{2,4} // between 2 and 4

Key takeaway: Pick strict quantifiers to reduce false-positive matches.

Anchors

Anchors match positions instead of characters. Start and end anchors help validate entire strings.

^hello   // starts with hello
world$   // ends with world
^\d{10}$ // exactly 10 digits

Key takeaway: Use anchors for strong validation patterns like IDs and phone numbers.

Simple Email Pattern

A practical starter pattern can validate common email-like strings for frontend checks.

\b\w+@\w+\.\w+\b

Key takeaway: Keep email regex practical; avoid over-complicated patterns unless required.

Advanced Regex Topics with Examples

Move to grouped extraction, lookaheads, and flag-driven parsing for real-world text processing.

Capturing vs Non-capturing Groups

Capturing groups store matched segments while non-capturing groups help grouping without adding unnecessary captures.

(https?)://(\w+\.\w+)   // capturing groups
(?:https?)://(\w+\.\w+) // first group non-capturing

Key takeaway: Use non-capturing groups when group data is not needed.

Lookaheads

Lookaheads validate future context without consuming characters. Useful for password and token rules.

^(?=.*[A-Z])(?=.*\d).{8,}$
// at least one uppercase, one digit, min length 8

Key takeaway: Lookaheads are strong for multi-rule validation in one pattern.

Global and Multiline Flags

Flags alter regex behavior. g finds all matches, i makes case-insensitive, and m changes line boundary behavior.

/error/gi
/^todo:/gm

Key takeaway: Always verify flags because they change result count and matching positions.

Log Parsing Example

Regex is effective for extracting IDs, levels, and timestamps from logs before analysis or monitoring.

const line = "[ERROR] 2026-02-23 userId=42";
const match = line.match(/\[(\w+)\]\s(\d{4}-\d{2}-\d{2}).*userId=(\d+)/);
console.log(match?.slice(1));

Key takeaway: Use grouped extraction for operational logs and analytics pipelines.