Matching re-occuring value within patterns

[muellpanda]

We propose a solution to evaluate our so called inter-observable patterns described in Issue #57.
I will try to describe what our code changes do:

• [1772-1782]: Since variables to describe inter-observables can only be defined within regular expressions, we adjusted the exitPropTestRegex-function. Whenever a regex containing a variable is successfully matched against an Observable object, we store the variable name, Observable ID and actual value the variable matched on in a nested dictionary (interobs_group_matches). Example: we have the pattern [file:name MATCHES '(?P<var>[a-z]{3})], a File-Observable with filename 'foo' and one with filename 'bar'. interobs_group_matches will look like this: {'var': {0: ['foo'], 1: ['bar']}}.
• [2147-2151]: We need to run our checks on all found bindings. Up to this point the actual matching process is unaltered and only those bindings are considered further which fulfill a pattern ignoring any variables.
• [2190-2209]: Check if any variables were matched, i.e. values exists within the dictionary. If not, return SDOs for bindings (original behaviour). If there are values, iterate over all found bindings. For each binding, only consider the relevant SDOs, i.e. create a copy of interobs_group_matches and remove all values of irrelevant SDOs. Then run the checks on this copy. If a check returns false, remove the current binding from found_bindings -- it does not fulfill the inter-observable pattern.
• [2165-2189]: The actual checks. If only one SDO is present, it is considered an invalid match if a variable matched on more than one unique value. When multiple SDOs are present, we look for intersections between the values for each variable; if no intersection exist, the variable did not match on the same values for these SDOs.

Our changes to the code are as transparent as possible to the original functionality of the matcher. We also added a small test to evaluate the functionality of inter-observable patterns.

[codecov-io]

Codecov Report

Merging #58 into master will increase coverage by 0.52%.
The diff coverage is 98.33%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   89.43%   89.95%   +0.52%     
==========================================
  Files          13       14       +1     
  Lines         937      996      +59     
==========================================
+ Hits          838      896      +58     
- Misses         99      100       +1

Impacted Files	Coverage Δ
stix2matcher/test/test_inter_observable_expr.py	100% <100%> (ø)
stix2matcher/matcher.py	87.62% <98.11%> (+0.71%)	⬆️

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[mdazam1942]

@muellpanda would you please resolve the conflicts. we would like to verify the changes and see if this can be merged