An expression of the form [.col.] matches the collating element col. A collating element is any single character, or any sequence
of characters that collates as a single unit. Collating elements may also be used as the end point of a range, for example: [.ae.]-c]
matches the character sequence "ae", plus any single character in the range "ae"-c, assuming that "ae" is treated as a single collating
element in the current locale.

Обьясните плиз что это такое и как его использовать.