About this tool
This tool prefixes each line of your text with a running number, or removes numbers you already have. It is handy for referencing code, quoting passages, building numbered lists or preparing text for a discussion where people need to point at specific lines. Everything happens in your browser and nothing is sent anywhere.
In Add mode the text is split on line breaks and each line gets its number, which starts at the Start value and increases by the Step you choose (so you can number 1, 2, 3… or 0, 5, 10…). The Separator sits between the number and the line — a period and space by default, but you can use a colon, a pipe, a tab or anything you like. Turn on zero-padding to align every number to the same width (for example 001, 002 … 100), and turn on skip blank lines to leave empty lines unnumbered so they do not consume a count. Switch to Remove mode to strip a leading number and its trailing separator from every line, undoing numbering from this or another source.
The output preserves your original line order and is fully copyable. Because it is pure string processing, it handles thousands of lines instantly and never alters the content of a line beyond adding or removing the number at its start.