> ## 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

> Extended regular expressions add OCR-error tolerance to formats: the [% %] brackets and LETTERS or DIGITS keywords match commonly misrecognized characters.

Formats are specified by means of extended regular expressions.

Extended regular expressions are [regular expressions](/flexi-capture/fls/appendix/reg-ex) with additional functionality enclosed between `[%` and `%]`. Extended regular expressions have the following additional features:

* One or more characters inside the brackets are supplemented with popular OCR errors. For example, `[%S%]` may allow S, \$, and 5.
* Special words inside `[%...%]` denote common character sets and their 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".

<Note>
  If search results are not satisfactory, open the **Advanced > Code** tab in the element's properties, check the regular expression to make sure there are no mistakes, and then copy it to the **Regular Expression** box and check the alphabet.
</Note>