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Regex Lab

Four questions about one piece of text: what matches, what a replacement would do, which lines survive an include and exclude list, and what the pattern actually says. Patterns shaped like a catastrophic backtrack are flagged before they run, because nothing in a browser can interrupt one that has started.

Runs entirely in this tab. No network requests, no logging, nothing leaves the page.

Pattern
//i
Test String
Matches
0.4 ms
Matches
4
Distinct
4
Lines hit
2
Words in text
26
Coverage
48.9%
of characters matched
Reach me at guan@railtown.ai or the old first.last@example.co.uk address. Support goes to help@example.com, not to noreply@example.com.
Captures
Line:ColMatch$1$2
1:13guan@railtown.aiguanrailtown.ai
1:47last@example.colastexample.co
2:17help@example.comhelpexample.com
2:42noreply@example.comnoreplyexample.com
Frequency
1guan@railtown.ai
1help@example.com
1last@example.co
1noreply@example.com
Starting points
12 patterns

Contact

Network

Code

Logs

Time

Prose

About this tool

01 · Four Analysis Modes

Unified workbench covering in-place syntax matches with capture groups, live replacement counters, dual-list line filtering (grep -v), and tokenized explanation trees.

Precision: Uses ECMAScript d flag indices.

02 · Catastrophic Backtracking Guard

Detects dangerous nested unbounded quantifiers (e.g. (a+)+b) statically before execution, holding dangerous patterns behind an explicit run confirmation.

Protection: Prevents tab lockup and lost drafts.

03 · Zero-Length Edge Cases

Correctly steps forward on zero-length matches (like \b) using Unicode code-point increments to prevent infinite loops and surrogate pair splits.

Robustness: Automatic 1.5-second execution deadline.

Operational Limits & Assumptions

  • · JavaScript Regex Dialect: Uses browser V8 RegExp semantics; PCRE features like possessive quantifiers or \K are not supported.
  • · Safety Budgets: Processing is bounded at 400,000 characters and 5,000 matches with execution timeout enforcement.
  • · Heuristic Warnings: ReDoD static checks target the most common polynomial/exponential structures; subtle backtracking patterns may bypass detection.
  • · Explain Limitations: Explanation view is a linear token scanner rather than a full AST compiler.

Found a case it gets wrong? Tell me about it .