...one of the most highly
regarded and expertly designed C++ library projects in the
world.

— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards

Often there is more than one way of matching a regular expression at a particular location, for POSIX basic and extended regular expressions, the "best" match is determined as follows:

- Find the leftmost match, if there is only one match possible at this location then return it.
- Find the longest of the possible matches, along with any ties. If there is only one such possible match then return it.
- If there are no marked sub-expressions, then all the remaining alternatives are indistinguishable; return the first of these found.
- Find the match which has matched the first sub-expression in the leftmost position, along with any ties. If there is only on such match possible then return it.
- Find the match which has the longest match for the first sub-expression, along with any ties. If there is only one such match then return it.
- Repeat steps 4 and 5 for each additional marked sub-expression.
- If there is still more than one possible match remaining, then they are indistinguishable; return the first one found.