String Processor (Buff)

String handling in ComfyUI is usually a string of disappointments: a node that uppercases, a node that replaces, a node that does one very specific thing you need exactly 2% of the time. StringProcessor bundles the common operations - take the first/last N characters, take the first/last N words - and then adds an escape hatch: a custom Python expression you type directly into the node. It's the Swiss-army string tool that covers the 98% you can't find a dedicated node for.

Realistically, you'll use it for filename cleanup, metadata tweaks, prompt formatting, or any place a value needs reshaping before it hits a text encoder or a save node. The "last 4 characters of this filename to get the extension" class of problem is its bread and butter.

How it works

Pick an operation, set n, and it does what it says - first_n_chars, last_n_chars, first_n_words, last_n_words. The word operations split on a delimiter you provide (default single space; set it to , for comma-separated lists) and rejoin with the same delimiter, so you're not left gluing fragments back together.

The interesting mode is custom_expression. You write Python using {s} as a placeholder for the input string, and the node evaluates it with s bound to the input. The tooltip ships a solid cheat-sheet: {s}.upper(), {s}.replace('old','new'), {s}[::-1] to reverse, {s}.split(',')[0] for CSV, even list comprehensions for title-casing. If the expression throws, the node returns an error string (Error in custom expression: ...) instead of crashing the graph, which is friendlier than most.

Fair warning, stated plainly: this runs eval() on your expression. It's a controlled scope (just s), and ComfyUI custom nodes are already arbitrary Python with full user privileges, so it's not a new risk surface - but don't paste expressions from untrusted strangers into it.

Inputs

• input_string - what you're processing (multiline allowed).
• operation - one of the five modes.
• n - character/word count for the slice operations.
• delimiter - only used by word ops.
• custom_expression - only used in custom mode.

Single output: processed_string.

Install

From BuffMcBigHuge's QoL pack - ComfyUI Manager (search "ComfyUI-Buff-Nodes") or:

cd ComfyUI/custom_nodes
git clone https://github.com/BuffMcBigHuge/ComfyUI-Buff-Nodes

Restart ComfyUI. No dependencies beyond stock.

Gotchas

The slice ops operate on the raw string, so first_n_words with a delimiter of " " treats runs of whitespace literally - double spaces can make it "skip" a word from your point of view. Empty input returns an empty string. And if a custom expression returns a non-string, it gets stringified silently, which is usually fine but occasionally surprising (a list becomes its repr). For the everyday filename-and-prompt surgery, the built-in ops cover 90% of needs and the expression field covers the other 10% with full Python. Hard to ask for more from one small node.