IW ReplaceString

IW ReplaceString does one thing: find a needle inside a haystack and swap in a value. In Python terms it's haystack.replace(needle, value) - no regex, no glob, no f-string parsing. It's the workhorse of prompt templating in the iwanders/ComfyUI_nodes pack, and the defaults are set up exactly for that job.

The template pattern

Out of the box the haystack defaults to {} and the needle to %REPLACE%. So the intended flow is: write a prompt with a placeholder, and let another node supply the real value. Maybe the value comes from IW StringFromInt (your seed as text), from IW JsonPickItem (one entry from a saved JSON list), or from an LLM node like this pack's OllamaGenerateString. Whatever the source, it lands on value and gets swapped in.

Where people get burned: str.replace replaces every occurrence of the needle, not just the first. If your template mentions %REPLACE% twice, both get filled - which is usually exactly what you want for a "make this consistent across the prompt" placeholder, but surprises you if you assumed only one slot existed. There's also no escaping, so a value that itself contains %REPLACE% is just inserted as-is (Python doesn't re-scan replaced text, so no infinite loop - the author avoided that particular foot-gun by accident of using the stdlib).

The json input mode

The fourth input, input_type, defaults to verbatim but can be set to json. In json mode, the value is parsed as JSON first: if it's a string, that string is used; if it's a list, its items are joined with spaces. That last bit is the quietly useful one - it means a JSON array of tags from a file or an API becomes a space-joined tag string for your prompt in one hop. If you want commas instead of spaces you'll need to transform the list yourself, but for Danbooru-style tag prompts, spaces are correct.

What you set

• haystack - the template, multiline.
• needle - the placeholder to replace.
• value - what replaces it; accepts wired input by default.
• input_type - verbatim or json, as above.

Output is a single STRING, ready to feed a CLIP Text Encode or another string node.

Installing

cd ComfyUI/custom_nodes
git clone https://github.com/iwanders/ComfyUI_nodes

Restart ComfyUI, or find iwanders in ComfyUI Manager. No dependencies, no model downloads. If you're building any workflow that varies one part of a prompt while keeping the rest fixed, this node is the boring, reliable way to do it.