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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:

 

1Find the leftmost match, if there is only one match possible at this location then return it.

2Find the longest of the possible matches, along with any ties. If there is only one such possible match then return it.

3If there are no marked sub-expressions, then all the remaining alternatives are indistinguishable; return the first of these found.

4Find 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.

5Find 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.

6Repeat steps 4 and 5 for each additional marked sub-expression.

7If there is still more than one possible match remaining, then they are indistinguishable; return the first one found.