Filter Lines (Keep or Remove Lines Containing)

A browser version of grep, head and tail for when you have a log file, a list or a CSV export and only some lines matter. Keep or delete every line that contains a word, starts or ends with something, is exactly a given value or matches a regular expression, with several terms at once. Or cut by position: the first or last N lines, a range of line numbers, or every Nth line. Paragraph mode does the same for blocks of text separated by blank lines.

How to use this tool

1. Paste the text.
2. Choose whether matching lines are kept or removed.
3. Choose how lines are matched and type the words or patterns, one per line, or set N and the line numbers for position-based filters.
4. Copy the filtered text; the note shows how many lines were kept and removed.

Formula

Kept line = (line matches) XOR (action is “remove”) · Every Nth from line S keeps lines S, S + N, S + 2N …

With several terms, “any” keeps a line that matches at least one term and “all” only lines that match every term. Matching ignores capital letters unless “match case” is ticked, and whole-word matching treats letters, digits and the underscore as word characters in every script. Regular expressions use JavaScript syntax and run on one line at a time. Patterns with a repeat inside a repeat, such as (a+)+, or a repeated choice whose options can match the same text, such as (a|aa)+, can take exponentially long and are refused unless you allow them.

Example

With the sample log, keeping lines that contain ERROR or WARN leaves 3 of 6 lines: the two payment timeouts and the disk warning. Switching the action to “remove” keeps the other three lines instead, and “first N” with N = 2 keeps just the first two log entries.