> ## Documentation Index
> Fetch the complete documentation index at: https://docs.abbyy.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Extended regular expressions

> Syntax guide for extended regular expressions in FlexiCapture for Invoices — special [% %] brackets and word sets like DIGITS account for common OCR errors.

Formats are specified by means of extended regular expressions.

## Additions to standard regular expressions

Extended regular expressions include the following additions:

1. Special brackets `[%` and `%]`. Characters inside the brackets are supplemented with common OCR errors. For example, `[%S%]` may allow `S`, `$`, and `5`.
2. Special words inside `[%...%]` for common character sets and OCR errors:
   * `LETTERS` — Capital Latin letters and characters commonly recognized as capital Latin letters.
   * `DIGITS` — Digits and characters commonly recognized as digits.
   * `LETTERSANDDIGITS` — Capital Latin letters, digits, and characters commonly recognized as capital Latin letters and digits.

For example, `[%DIGITS%]{9}` specifies nine consecutive digits or common OCR errors for digits, such as `OI234Sb7B9`.