The (undef rule :tag) combinator lets a user "scope" tagged captures.
After the rule has matched, all captures with tag :tag can no longer be
refered to by their tag. However, such captures from outside
rule are kept as is. If no tag is given, all tagged captures from rule
are unreferenced. Note that this doesn't `drop` the captures, merely
removes their association with the tag. This means subsequent calls to
`backref` and `backmatch` will no longer "see" these tagged captures.
These capture the line and column number of the current position
in the matched text. This is useful for error reporting as well
as indentation checking.
This works by lazily creating an index on first use that stores all
newline character indices in order. We can then do a binary search on
this to get both line number and column number in log(n) time.
This is good enough for most use cases and doesn't slow down the common case at all
- these will not be commonly used patterns in a hot loop so it is not worth to try and
optimize this at all. Constant time look up should be possible but at
the cost of complicating code and slowing down all matching to check for
new lines.
