Posix +Text.Regex -package
Module that provides the Regex backend that wraps the c posix regex api. This is the backend being used by the regex-compat package to replace Text.Regex
The Text.Regex.Posix module provides a backend for regular expressions. If you import this along with other backends, then you should do so with qualified imports, perhaps renamed for convenience.
If the =~ and =~~ functions are too high level, you can use the compile, regexec, and execute functions from importing either Text.Regex.Posix.String or Text.Regex.Posix.ByteString. If you want to use a low-level CString interface to the library, then import Text.Regex.Posix.Wrap and use the wrap* functions.
This module is only efficient with ByteString only if it is null terminated, i.e. (Bytestring.last bs)==0. Otherwise the library must make a temporary copy of the ByteString and append the NUL byte.
A String will be converted into a CString for processing. Doing this repeatedly will be very inefficient.
Note that the posix library works with single byte characters, and does not understand Unicode. If you need Unicode support you will have to use a different backend.
When offsets are reported for subexpression captures, a subexpression that did not match anything (as opposed to matching an empty string) will have its offset set to the unusedRegOffset value, which is (-1).
Benchmarking shows the default regex library on many platforms is very inefficient. You might increase performace by an order of magnitude by obtaining libpcre and regex-pcre or libtre and regex-tre. If you do not need the captured substrings then you can also get great performance from regex-dfa. If you do need the capture substrings then you may be able to use regex-parsec to improve performance.