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Regex Engine - Free Online Regex Engine Visualizer Tool | IconVault

Watch a backtracking regex engine work. Type a pattern and test string, then step through every match attempt, backtrack and capture. Free, in your browser.

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Regex Engine

Watch backtracking happen token by token

5 of 5 free uses left
//

Presets

1 / 64
Speed4/s

5 of 5 free copies left - everything runs locally in your browser.

Pattern tokens

(\w+)@(\w+)\.com

Test string (caret = engine position)

mail␣sam@example.com␣today
Step 1. engine: try a match starting at index 0

Engine log

1. engine: try a match starting at index 0
2. enter group 1
3. class \w matches 'm' at 0
4. class \w matches 'a' at 1
5. class \w matches 'i' at 2
6. class \w matches 'l' at 3
7. class \w vs ' ' at 4 - no match
8. \w+: greedy, try 4 repetitions
9. group 1 captured "mail"
10. '@' vs ' ' at 4 - no match
11. engine: try a match starting at index 1
12. enter group 1
13. class \w matches 'a' at 1
14. class \w matches 'i' at 2
15. class \w matches 'l' at 3
16. class \w vs ' ' at 4 - no match
17. \w+: greedy, try 3 repetitions
18. group 1 captured "ail"
19. '@' vs ' ' at 4 - no match
20. engine: try a match starting at index 2
21. enter group 1
22. class \w matches 'i' at 2
23. class \w matches 'l' at 3
24. class \w vs ' ' at 4 - no match
25. \w+: greedy, try 2 repetitions
26. group 1 captured "il"
27. '@' vs ' ' at 4 - no match
28. engine: try a match starting at index 3
29. enter group 1
30. class \w matches 'l' at 3
31. class \w vs ' ' at 4 - no match
32. \w+: greedy, try 1 repetition
33. group 1 captured "l"
34. '@' vs ' ' at 4 - no match
35. engine: try a match starting at index 4
36. enter group 1
37. class \w vs ' ' at 4 - no match
38. \w+ could not satisfy the rest of the pattern
39. group 1 failed
40. engine: try a match starting at index 5
41. enter group 1
42. class \w matches 's' at 5
43. class \w matches 'a' at 6
44. class \w matches 'm' at 7
45. class \w vs '@' at 8 - no match
46. \w+: greedy, try 3 repetitions
47. group 1 captured "sam"
48. match '@' at index 8
49. enter group 2
50. class \w matches 'e' at 9
51. class \w matches 'x' at 10
52. class \w matches 'a' at 11
53. class \w matches 'm' at 12
54. class \w matches 'p' at 13
55. class \w matches 'l' at 14
56. class \w matches 'e' at 15
57. class \w vs '.' at 16 - no match
58. \w+: greedy, try 7 repetitions
59. group 2 captured "example"
60. match '.' at index 16
61. match 'c' at index 17
62. match 'o' at index 18
63. match 'm' at index 19
64. match found: "sam@example.com" at 5-20

Match result

Matched "sam@example.com"

span 5-20

group 1"sam"
group 2"example"

Simplified model: greedy matching, no lazy quantifiers, no lookarounds, no backreferences. Real engines (V8, PCRE) share this backtracking core.

About this tool

IconVault's Regex Engine visualizer opens up the black box of regular expressions. Type a pattern and a test string, and a simplified NFA-style simulator replays the match attempt token by token: every try, every successful step, every failure and every backtrack, with capture groups tracked along the way.

You can step forward and backward, play the sequence automatically, and see exactly where a pattern slows down or fails. It is a brilliant way to finally understand catastrophic backtracking. The whole thing runs free in your browser with no signup.

Frequently asked questions

Popular searches

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