Utilities that process regular expressions shall support Basic Regular Expressions and Extended Regular Expressions as specified in ISO POSIX (2003), with the following exceptions:
Range expression (such as [a-z]) can be based on code point order instead of collating element order.
Equivalence class expression (such as [=a=]) and multi-character collating element expression (such as [.ch.]) are optional.
Handling of a multi-character collating element is optional.
This affects at least the following utilities:
regexec() (see regexec)