What is run-length encoding?

Run-length encoding is a simple lossless compression scheme that replaces a run of identical consecutive symbols with a single count and one copy of the symbol. AAAB becomes 3A1B.

Does RLE always make data smaller?

No. RLE only helps when the data has long runs of repeated symbols, such as simple images or whitespace. For random or already-varied text it can make the output longer, since every single character becomes a count plus the character.

How does this tool format the encoding?

It uses count-then-character pairs with no separators, for example AAABBC encodes to 3A2B1C. Every run, including runs of length one, is written with an explicit count so decoding is unambiguous.

Can the decoder handle multi-digit counts?

Yes. A count like 12 is read fully before the following character, so 12X expands to twelve X characters. The decoder reads all leading digits as the count for the next symbol.

What happens with digits in the original text?

Because counts are numeric, plain digits in the source can be ambiguous to decode. Encoding still works (each digit run gets a count), but if your original data contains digits, round-tripping relies on every run being written as count+character, which this tool always does.

Run-Length Encoding (RLE)

What this tool does

Run-length encoding (RLE) is one of the oldest and simplest lossless compression methods. It replaces a “run” of identical consecutive characters with a single count followed by the character. This tool both encodes text into RLE form and decodes RLE back to the original.

How it works

Encoding scans the string left to right, counting how many times the current character repeats consecutively. When the character changes (or the string ends), it writes the count followed by that character, then resets the counter. Every run — even a run of length one — is written with an explicit count, so AAABBC becomes 3A2B1C. This explicit-count rule keeps decoding unambiguous.

Decoding reads a sequence of count-then-character pairs. It accumulates all leading digits into a number, then repeats the next character that many times. So 12X expands to twelve Xs, and 3A2B1C expands back to AAABBC.

Example

Input:   WWWWWWWWWWWWBWWWWWWWWWWWWBBB
Encoded: 12W1B12W3B

That run of 27 characters compresses to 10 — a clear win because the data is highly repetitive.

Notes

RLE shrinks data only when runs are long; on varied text it can grow the output, since each isolated character costs an extra count digit.
The tool counts by Unicode code point, so emoji and accented characters are treated as single symbols.
For correct round-tripping, keep the count-character pairing intact when editing encoded text by hand.