match_at: implementation of String#match_at, Regexp#match_at

Regular expressions are useful but current Ruby lacks one feature: to match a regular expression against particular position of a String. This functionality is in fact implemented in the regular expression engine, but have not been exposed so far. This is a very tiny extension library to give you that feature.

Your basic usage of regular expressions in ruby is like this:

md = /bar/.match "foobarbaz" # => #<MatchData "bar">

# 0 1 2 3 4 5 6 7 8
# f o o b a r b a z
#       |---|
#       match

md.pre_match  # => "foo"
md.post_match # => "baz"
md.begin(0)   # => 3
md.end(0)     # => 5

Here, the regular expression matches at the middle of the parameter string. Normal behaviour of a regular expression is to search for leftmost & longest string that fits the pattern.

But that is not always what you want. When you want to split a long string into a series of tokens, it is usually not optimal to scan again and again from the beginning.

... f o o b a r b a z ...
    ---> |
         suppose you have already scanned here.
         you want to start from this exact position...

This kind of situation has formerlly been tackled by the StringScanner standard library. That is still okay today. However strscan is not "fluent"; for instance you can't get a MatchData with it.

Luckily as I wrote above the functionality is already there. This library is to let you use it.

md = /bar/.match_at "foobarbaz", 3 # => #<MatchData "bar">

# 0 1 2 3 4 5 6 7 8
# f o o b a r b a z
#       |---|
#       match

md.pre_match  # => "foo"
md.post_match # => "baz"
md.begin(0)   # => 3
md.end(0)     # => 5