How is this different from Slice a String?

This tool takes a start position and a length. Slice a String instead takes a start and end index pair. Both extract a substring; they just describe the range differently.

What does a negative start position do?

It counts backward from the end of the string, similar to negative indices in slice(), so a start of -3 begins three characters before the end.

What if the length exceeds the remaining characters?

The extraction simply stops at the end of the string; no error is raised, and you get however many characters remain.

Substring Extractor Online - Start Position + Length

Overview

Introduction

Some extraction tasks are more naturally described as 'give me N characters starting here' rather than 'give me everything between these two indices'. This tool covers that case directly.

It runs entirely client-side, so nothing you paste is ever uploaded to a server.

What Is Substring Extractor?

A substring extractor that takes a start position and a length, returning exactly that many characters (or fewer, if the string ends first).

It runs entirely client-side as part of this site's String Tools collection, so nothing you paste is ever uploaded to a server.

How Substring Extractor Works

The tool resolves a negative start position relative to the string's end, then extracts `length` characters from that point using code-point-aware slicing.

The transformation happens synchronously in JavaScript the moment you type, with no network round trip involved.

When To Use Substring Extractor

Use it when you know a fixed-width field's starting position and length, common when parsing fixed-width legacy data formats.

It's a fast way to get the answer without opening a code editor, a REPL, or writing a one-off script just to check.

Often used alongside String Slicer and String Truncator.

Features

Advantages

Start-plus-length is often more natural than an index pair for fixed-width field extraction.
Operates on Unicode code points, keeping multi-byte characters intact.

Limitations

Doesn't error on an out-of-range length; it silently returns whatever characters are available.

Examples

Extracting a fixed-width field

Input

20260722ADA36

Output

ADA

With start=8 and length=3, exactly three characters are extracted starting at position 8.

Best Practices & Notes

Best Practices

Use Slice a String instead if you already have a start/end index pair rather than a length.

Developer Notes

A negative start is first resolved to a non-negative index via `Math.max(0, chars.length + start)`, then the extraction is a straightforward slice(from, from + length) over the code-point array.

Substring Extractor Use Cases

Parsing a fixed-width field from legacy flat-file data
Extracting a known-length code from a longer identifier
Testing substr()-style extraction logic

Common Mistakes

Confusing this with Slice a String's index-pair semantics.

Tips

Use Slice a String instead if you're thinking in terms of two index positions rather than a start and length.

Substring Extractor