Regular Expression Calculator

What the Regular Expression Tester calculates

Tests a regular expression against a piece of text with a linear-time engine that cannot hang: it simulates all match paths at once instead of backtracking, so hostile patterns like (a+)+b finish in milliseconds. Matching is leftmost-longest, which can pick a longer match than JavaScript would. Backreferences and lookaround are refused rather than faked.

How to use this Regular Expression Tester

Enter Regular expression, Text to search and Work budget (simulation steps). Match found updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.

How the result is worked out

Open “How this is worked out” beside the result for the step-by-step calculation behind Match found, First match and Match starts at index.

The method follows Ken Thompson, "Programming Techniques: Regular expression search algorithm", Communications of the ACM 11(6), 1968 and Russ Cox, "Regular Expression Matching Can Be Simple And Fast".

A phone-shaped pattern finds its span example

With Regular expression: \d{3}-\d{4}, Text to search: call 555-0199 today, the calculator returns Match found: yes, First match: 555-0199, Match starts at index: 5. This example is checked against an independent source in the test suite.

What you can compare

Change one input at a time to see how it affects Match found, First match and Match starts at index. The inputs you can vary are Regular expression, Text to search and Work budget (simulation steps).

Understanding Match found, First match and Match starts at index

This tester is deliberately not JavaScript-compatible on two points: it matches leftmost-longest rather than leftmost-first, and it refuses backreferences and lookaround outright. A pattern that works here may behave differently in a backtracking engine, and one that hangs a backtracking engine will finish here.