Edit distance of fuzzy match

[mrabarnett]

Original report by Anonymous.

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I'm using Regex for fuzzy match, and the application needs to compute the exact edit distance between the regex and the matched string. I can't find this feature in the document. It would be very useful to include the exact number of insertions, replacements and deletions in the returned match object.

Thank you for providing such powerful regex library!

[mrabarnett]

Original comment by Anonymous.

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You'll need to bear in mind that it would report the total number of substitutions, insertions and deletions that it did in order for it to match, which would not necessarily be the minimum.

For example, when trying this:

>>> match(r'(?:cats|cat){e<=1}', 'cat')
<regex.Match object; span=(0, 3), match='cat', fuzzy_counts=(0, 0, 1)>

it would report that it matches with one deletion.

This is because, when faced with alternatives, it tries them in sequence, so it will try to match "cats" first, which it can do with one deletion (the "s").

It doesn't try every possibility by default (that could take a long time!), so it won't necessarily find the minimum edit distance.

However, you could use the ENHANCEMATCH or BESTMATCH flag to look for a better fit:

>>> match(r'(?e)(?:cats|cat){e<=1}', 'cat')
<regex.Match object; span=(0, 3), match='cat'>

I'm not sure what the best arrangement would be on the match object. A single attribute called, say, 'fuzzy_counts' as above? Three separate attributes called 'substitutions', 'insertions' and 'deletions'?

[mrabarnett]

Original comment by Anonymous.

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I think the fuzzy_count attribute in the example will suffice, thanks!

Could you explain a little about the difference of the oridinary, enhancematch and best match? Do they use different search algorithms? It's better to give the running complexity in the documentation.

[mrabarnett]

Original comment by Anonymous.

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The feature isn't available at the moment, but should be in the next release. I'm right in the middle of another issue at the moment.

There's a brief explanation of the ENHANCEMATCH and BESTMATCH flags in the "Flags" section here:

https://code.google.com/p/mrab-regex-hg/wiki/GeneralDetails

[mrabarnett]

Original comment by Anonymous.

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Thanks for your efforts!

[mrabarnett]

Original comment by Anonymous.

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Added in regex 2014.04.10.

[mrabarnett]

Original comment by Anonymous.

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Hi! Could I check if this is expected behavior or if I'm misunderstanding something?

1. fuzzy_counts don't include substitutions when ENHANCEMATCH or BESTMATCH is on

   re.match(r'(?:cats|cat){e<=1}', 'caz')
   <regex.Match object; span=(0, 3), match='caz', fuzzy_counts=(1, 0, 0)>

   re.match(r'(?e)(?:cats|cat){e<=1}', 'caz')
   <regex.Match object; span=(0, 3), match='caz'>

   re.match(r'(?b)(?:cat){e<=1}', 'caz')
   <regex.Match object; span=(0, 3), match='caz'>

2. Insertions are not always included in fuzzy_counts

   re.match(r'(?:cats){e<=2}', 'c ats')
   <regex.Match object; span=(0, 5), match='c ats', fuzzy_counts=(1, 1, 0)>
   #I understand the edit distance may not be minimum, assume it is matching 'c ats' to '-cats' or 'c-ats' or some such thing?

   re.match(r'(?b)(?:cats){e<=2}', 'c ats') #minimum edit distance
   <regex.Match object; span=(0, 5), match='c ats', fuzzy_counts=(0, 1, 0)>

   re.match(r'(?b)(?:cats){e<=2}', 'c a ts') #why edit distance 0?
   <regex.Match object; span=(0, 6), match='c a ts'>

[mrabarnett]

Original comment by Anonymous.

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Really you should've created a new issue for a bug because this issue is for an enhancement that has been completed.

It is a bug, though.

[mrabarnett]

Original comment by Anonymous.

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Fixed in regex-2014.05.23.