Fix RegexExtract crashing/returning None on non-participating capture groups

[vidigoat]

Problem

The RegexExtract node (Extract Text) crashes or leaks a non-string value when a requested capture group didn't participate in the match.

In "First Group" and "All Groups" modes the code only checks len(match.groups()) >= group_index, then calls match.group(group_index). When the group is optional or on a non-taken alternation branch, match.group() returns None:

# "First Group"
re.search(r"(\d+)-(\d+)|(\w+)", "hello")   # matches via the (\w+) branch
match.group(1)                              # -> None  (returned from a String output)

# "All Groups"
# appends None, then join() raises:
# TypeError: sequence item 0: expected str instance, NoneType found  (workflow aborts)

So a common pattern with an alternation or optional group either silently sends None into string-consuming nodes ("First Group") or hard-crashes prompt execution ("All Groups").

Fix

Guard both branches with is not None, so a non-participating group yields "" (matching the node's existing "no match" behaviour). Normal matches are unchanged.

Added tests-unit/comfy_extras_test/nodes_string_test.py covering both modes with non-participating groups plus the normal paths; it fails before this change (None return + TypeError) and passes after. The full test runs green against the real node.

---

Disclosure: I used an AI assistant while working on this; I reproduced the bug, wrote the fix and test, and verified everything myself.

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🧠 Learnings (6)

📚 Learning: 2026-02-21T14:01:41.482Z

Learnt from: pythongosssss
Repo: Comfy-Org/ComfyUI PR: 12555
File: comfy_extras/nodes_glsl.py:719-724
Timestamp: 2026-02-21T14:01:41.482Z
Learning: In PyOpenGL, bare Python scalars can be accepted for 1-element array parameters by NumberHandler. This means you can pass an int/float directly to OpenGL texture deletion (e.g., glDeleteTextures(tex)) without wrapping in a list. Verify function-specific expectations and ensure types match what the OpenGL call expects; use explicit lists only when the API requires an array.

Applied to files:

• tests-unit/comfy_extras_test/nodes_string_test.py
• comfy_extras/nodes_string.py

📚 Learning: 2026-07-13T17:26:10.852Z

Learnt from: silveroxides
Repo: Comfy-Org/ComfyUI PR: 14922
File: tests-unit/comfy_extras_test/test_qwen_visual_fusion.py:3-7
Timestamp: 2026-07-13T17:26:10.852Z
Learning: In Comfy unit tests that import CUDA-dependent modules (e.g., tests that need `comfy_extras/nodes_qwen.py`), set the global `cli_args.cpu = True` at module load time *before* importing `comfy_extras/nodes_qwen.py` when CUDA is unavailable. Do not rely on `pytest` fixtures or `monkeypatch` for this kind of import-time behavior, because fixtures run after the test module is imported/collected and the CUDA code would already execute. The goal is to prevent import-time CUDA initialization and keep the tests runnable in CPU-only environments.

Applied to files:

• tests-unit/comfy_extras_test/nodes_string_test.py

📚 Learning: 2026-03-04T14:05:31.426Z

Learnt from: jtydhr88
Repo: Comfy-Org/ComfyUI PR: 12757
File: comfy_extras/nodes_custom_sampler.py:1069-1089
Timestamp: 2026-03-04T14:05:31.426Z
Learning: In the ComfyUI sampling pipeline, treat percent_to_sigma(0.0) as a sentinel value (999999999.9) that means starting from pure noise. This is consistent with BasicScheduler via calculate_sigmas. The SamplingPercentToSigma node’s return_actual_sigma flag differentiates this sentinel from sigma_max. Reviewers should not flag CurveToSigmas or similar nodes that rely on percent_to_sigma as bugs; downstream samplers are expected to handle the sentinel correctly. When reviewing related sampling-related code, assume this sentinel semantics unless there is explicit handling for a real sigma_max.

Applied to files:

• comfy_extras/nodes_string.py

📚 Learning: 2026-04-04T13:29:15.653Z

Learnt from: kijai
Repo: Comfy-Org/ComfyUI PR: 13258
File: comfy_extras/nodes_frame_interpolation.py:151-189
Timestamp: 2026-04-04T13:29:15.653Z
Learning: In this ComfyUI codebase, node `execute()` inference is already run under a global `torch.inference_mode()` context established in the execution engine (e.g., `execution.py` around line ~732). During review, avoid recommending changes that wrap node inference loops in `torch.inference_mode()`—it is already applied, so such suggestions are likely redundant.

Applied to files:

• comfy_extras/nodes_string.py

📚 Learning: 2026-05-09T18:40:40.199Z

Learnt from: kijai
Repo: Comfy-Org/ComfyUI PR: 13813
File: comfy_extras/nodes_wandancer.py:868-872
Timestamp: 2026-05-09T18:40:40.199Z
Learning: When building video/temporal decoding nodes that call ComfyUI’s VAE.decode (comfy/sd.py), leverage VAE.decode’s existing VRAM-aware chunking along dim 0. Reshape or transpose the latent so the temporal dimension T is folded into dim 0 (e.g., transform a latent of shape [B, T, C, H, W] into [B*T, C, H, W] before calling vae.decode). This lets VAE.decode do chunked decoding without needing an explicit per-frame loop inside the node itself.

Applied to files:

• comfy_extras/nodes_string.py

📚 Learning: 2026-05-20T00:10:14.673Z

Learnt from: Pauan
Repo: Comfy-Org/ComfyUI PR: 13997
File: comfy_extras/nodes_string.py:12-25
Timestamp: 2026-05-20T00:10:14.673Z
Learning: In the ComfyUI `comfy_extras/` codebase, some nodes intentionally ship with a default input string that references parameters that may not yet be connected. If the default would raise a `KeyError` (e.g., examples like `MathExpression` default `a + b`, or `StringFormat` default `{a}` with `min=0` and autogrow inputs), treat it as an intentional “hint default” UX pattern, not a bug. During review, do not flag this behavior or recommend changing `min` to `1` or altering the default to an empty string solely to avoid the `KeyError`.

Applied to files:

• comfy_extras/nodes_string.py

🔇 Additional comments (2)

comfy_extras/nodes_string.py (1)

352-359: LGTM!

Also applies to: 365-371

tests-unit/comfy_extras_test/nodes_string_test.py (1)

1-8: LGTM!

Also applies to: 11-20, 23-40

---

📝 Walkthrough

Walkthrough

Updated RegexExtract to check capture-group values before returning or collecting them. Non-participating groups now produce an empty string in “First Group” mode and are omitted in “All Groups” mode. Added isolated unit tests covering module mocking, non-participating groups, mixed group participation, and normal extraction.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: fixing RegexExtract handling of non-participating capture groups.
Description check	✅ Passed	The description directly explains the bug, the fix, and the added tests, so it is clearly related to the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[vidigoat]

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