Regex Alternation Order
I set up a complex regex to extract data from a page of text. For some reason the order of the alternation is not what I expect. A simple example would be:
((13th|(Executive |Residential)|((\w+) ){1,3})Floor)
Put simply I am trying to either get a floor number, a known named floor and, as a back-up, I capture 1-3 unknown words followed by floor just in case to review later (I in fact use a groupname to identify this but didn't want to confuse the issue)
The issue is if the string is
on the 13th Floor
I don't get 13th Floor I get on the 13th Floor which seems to indicate it is matching the 3rd alternation. I'd have expected it to match 13th Floor. I set this up specifically (or so I thought) to prioritize the types of matches and leave the vague ones for last only if the others are missed. I guess they weren't kidding when they said Regex is greedy but I am unclear how to set this up to be 'greedy' and behave the way I want.
Well an automaton is worth a 1000 words:
Your problem is that you're using a greedy \w+ sub-regex in your alternation. Because as @rigderunner is stating in his comment, a NFA is matching the longest leftmost substring, the \w+ will always match anything that comes before Floor, whether it is a series of words, or 13th or Executive or Residential or the three of them. The parenthesis are not changing how the alternation behaves.
So the worst case scenario it matches that you don't want it to match is:
xxxx yyyy zzz tttt Floor
The problem with your regex is that you expect to do something that actual regular expressions can't do: you're expecting it to match words if the alternatives did not work out. Because a regular language can't keep track of status, regular regex can't express this.
I'm actually not sure if using some kind of look ahead could help you do this in one regex, and even if you can, you'll end up with a very complicated, unreadable and maybe even not efficient regex.
So you may prefer to use two regex instead, and get the groups from the second regex in case the first one failed:
((13th|Executive|Residential) +Floor)
and if there's no match
((\w+ +){1:3}Floor)
N.B.: to avoid repeating myself, please have a look at that other answer where I give a list of interesting resources lecturing on regex and NFA. That will help you understand how regex actually works.
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