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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:
| 1 | Find the leftmost match, if there is only one match possible at this location then return it. |
| 2 | Find the longest of the possible matches, along with any ties. If there is only one such possible match then return it. |
| 3 | If there are no marked sub-expressions, then all the remaining alternatives are indistinguishable; return the first of these found. |
| 4 | 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. |
| 5 | 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. |
| 6 | Repeat steps 4 and 5 for each additional marked sub-expression. |
| 7 | If there is still more than one possible match remaining, then they are indistinguishable; return the first one found. |
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