Debugging regex matches

Debug mode walks through how a pattern relates to a subject, step by step. It is designed for learning and explaining matches (and misses), not as a cycle-accurate simulator of the engine’s internal NFA.

Open Debug

Toolbar or right panel tab: Debug
Keyboard: Ctrl+7 (⌘+7 on macOS)
While debugging: F10 (or Ctrl+→) step forward · F11 (or Ctrl+←) step back
Ctrl+Enter rebuilds the session from the current pattern, subject, and options

Engine support

Testing engine	Debug
.NET (`dotnet`)	Supported
PCRE2	Not yet — clear message in the panel
JavaScript (Jint)	Not yet — clear message in the panel

Any flavor that uses the .NET engine for testing (including approximate flavors such as Python/Java when they map to dotnet) can use Debug. Flavors on PCRE2 or JavaScript show:

Step-through Debug is currently available only for the .NET engine…

Switch the flavor to .NET (or another .NET-backed flavor) to enable stepping.

What you see

UI element	Meaning
Step N / M	Current position in the walk-through
Kind badge	Start, Overview, Pattern, Attempt, Capture, Match, Fail, Done, …
Status	OK / Fail / Info
Explanation	Human-readable description of the step
Pattern focus	Fragment of the regex under consideration (also selected in the center editor)
Subject focus	Fragment of the subject under consideration (highlighted in the Debug subject editor)
Step list	Click any row to jump to that step
Approach note	Reminds you this is an educational overlay on real Match results

Controls: ↺ first step · ◀ back · ▶ forward · ⇥ last step · Refresh rebuild.

How it works (approach)

RegexCraft does not re-implement the full .NET regex engine for Debug. Instead it builds a hybrid educational session:

Runs a real .NET Match (same results as Test mode).
Walks the Analysis Tree (pattern structure) left to right.
Overlays real match offsets and capture groups onto those steps.

So:

Match counts, group values, and positions match live Test for the .NET engine.
Intermediate “applying this construct” steps teach structure; some subject ranges for meta-heavy fragments are estimated within the successful match span.
Failed searches explain the pattern structure and report “no match” from the real engine.

Tips

Use the Analysis Tree in the center column alongside Debug — clicking a node still selects that range in the pattern.
Keep Test results in mind: Debug re-runs Match when you open or refresh Debug so both stay aligned.
For multi-match subjects, the session walks each match (attempt → pattern nodes → captures → success).
Invalid patterns produce Error steps with the engine’s message; fix the pattern, then Refresh.

Limitations (1.1)

No step-through for PCRE2 / JavaScript yet (architecture allows adding more engines later).
Not a perfect backtracking / NFA simulation (no guaranteed “engine cycle” fidelity).
Lookarounds, complex quantifier retries, and catastrophic backtracking are not simulated step-by-step.
Subject positions for some abstract nodes are approximate within the match.