ImageGear for C and C++ on Windows v19.10 - Updated
Predefined Sets
User Guide > How to Work with... > OCR > Technical Specifications > Regular Expressions > Anatomy of a Regular Expression > Predefined Sets

You compose a predefined set by using the backslash character together with an alphabetical character. Lowercase letters indicate inclusive sets, character classes composed of the characters having a special feature; capital letters denote the negated class of their corresponding lowercase set, composed of all the characters not having that special feature.

Predefined character sets represented by lowercase characters can be included within user-defined sets. For example '[\da-fA-F]' describes hexadecimal characters. Of course, you cannot define a range with either end being a predefined set.