About

Prepending a consistent prefix to each line of a text block is a routine operation in data preparation, log formatting, and code commenting. Manual editing across hundreds of lines introduces typos, inconsistent spacing, and wasted time. This tool processes n lines in O(n) time, applying a user-defined prefix string P with a configurable separator S to produce output lines of the form P + S + L_i. It handles edge cases: empty lines, trailing whitespace, and optional zero-padded line numbering. The tool assumes UTF-8 input and preserves original line breaks.

Formulas

Each input line L_i is transformed by concatenating the prefix P, an optional separator S, and the original line content:

Output_i = P + S + L_i

When line numbering is enabled with zero-padding width w, the prefix becomes a composite:

Output_i = P + padStart(i, w, 0) + S + L_i

Where P is the user-defined prefix string. S is the separator character (space, tab, colon, or none). L_i is the i-th line of input text. w is computed as floor(log₁₀(n)) + 1, where n is the total line count. When the skip-empty-lines option is active, lines where trim(L_i).length = 0 are passed through unmodified.

Reference Data

Prefix Use Case	Typical Prefix	Separator	Context
SQL Comments	--	Space	Disabling SQL statements in bulk
Python Comments	#	Space	Commenting out code blocks
C/Java Comments	//	Space	Single-line comment blocks
Markdown Lists	-	Space	Converting plain lines to bullet lists
Ordered Lists	1.	Space	Creating numbered Markdown lists
Email Quoting	>	Space	Quoting replied text in email or Markdown
Log Tagging	[INFO]	Space	Categorizing log entries by severity
Error Tagging	[ERROR]	Space	Marking error-level log lines
CSV Field Add	value,	None	Prepending a column to CSV rows
Indentation	(4 spaces)	None	Adding indent level to code blocks
Tab Indentation	\t	None	Adding tab-based indentation
HTML List Items	<li>	None	Wrapping lines in list item tags
Bash Echo	echo	Space	Converting text lines to shell echo commands
Git Commit Prefix	fix:	Space	Conventional commit message formatting
Timestamp	2024-01-15	Space	Date-stamping log or journal entries
Namespace	app.	None	Adding namespace prefix to config keys
TODO Marker	TODO:	Space	Marking lines as action items
Checkbox (MD)	- [ ]	Space	Creating Markdown task lists
Arrow Indicator	→	Space	Visual indicators in documentation
Custom Label	[REVIEW]	Space	Flagging lines for peer review

Frequently Asked Questions

By default, the prefix is applied to every line including empty ones. When the "Skip Empty Lines" option is enabled, lines consisting solely of whitespace (after trimming) are passed through without modification. This prevents output like a lonely prefix on otherwise blank lines, which is common when commenting out code blocks that contain spacing.

The tool normalizes all line endings to LF (\n) during processing, which is the standard for web-based text handling. If your workflow requires CRLF (Windows-style), paste the output into an editor that converts line endings, or use a secondary tool. The normalization ensures consistent behavior across macOS, Linux, and Windows inputs.

The padding width w is calculated as floor(log₁₀(n)) + 1, where n is the total number of non-skipped lines. For example, 99 lines produce width 2 (01, 02, ... 99), while 100 lines produce width 3 (001, 002, ... 100). This keeps columnar alignment consistent.

Yes. The separator dropdown includes a Tab option that inserts a literal tab character (\t) between the prefix and the line content. You can also select "None" for zero-width joining, which is useful for namespace prefixes like "app." where no space is desired between the prefix and the original text.

The tool processes text synchronously in O(n) time. On modern browsers, inputs up to approximately 100,000 lines (roughly 5 MB of text) process in under 200ms. Beyond that threshold, browser tab memory limits become the bottleneck rather than the algorithm. For extremely large files, consider splitting the input or using a command-line tool like sed or awk.

The "Trim Lines" option removes leading and trailing whitespace from each line L_i before the prefix is prepended. This is applied after the skip-empty-lines check. It is useful when pasting text from sources with inconsistent indentation, ensuring the prefix aligns cleanly at the start of each line's actual content.