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Topic: Regex and substring (Read 1692 times) |
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Barukh
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Regex and substring
« on: Oct 18th, 2015, 8:53am » |
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A friend of mine asked me the following intriguing question:
Given a regular expression R and a string S.
Devise an algorithm to determine whether there exists another string S' s.t.
1. S is a substring of S'.
2. S' matches R.
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towr
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Re: Regex and substring
« Reply #1 on: Oct 18th, 2015, 1:11pm » |
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Build the finite state automaton corresponding to R
Try each state as starting state to check if S can be a substring of any string matching R
Work backwards/forwards from start/end of S to find a fitting S'.
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| « Last Edit: Oct 18th, 2015, 1:11pm by towr » |
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Barukh
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Re: Regex and substring
« Reply #2 on: Oct 19th, 2015, 9:12am » |
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Nice! But...
So, it all boils down to efficiently building an FA for the regex?
Can it be efficient always?
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playful
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Re: Regex and substring
« Reply #3 on: Apr 24th, 2016, 3:03am » |
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Here is how I would get started, taking an example.
Bracing myself for a semantic discussion on the meaning of "regular expression".  The title of the puzzle does say "regex".
Example
1. regex pattern R: \d+@hello
2. string S: hello
Idea for Algorithm
For each position p in the pattern R
...Rp = subpattern from p to end of R
...Temp regex Rt = ''
...For each token t in Rp
......Rt += t
......If Rt matches S
.........Return success
Return failure
Tracing this example:
- Position p=0
-- Try to match "hello" with \d+ => Fail
-- Try to match "hello" with \d+@ => Fail
-- Try to match "hello" with \d+@h => Fail
...
-- Try to match "hello" with \d+@hello => Fail
- Position p=1
-- Try to match "hello" with @ => Fail
-- Try to match "hello" with @h => Fail
...
-- Try to match "hello" with @hello => Fail
- Position p=2
-- Try to match "hello" with h => Fail
-- Try to match "hello" with he => Fail
...
-- Try to match "hello" with hello => Succeed
Example string S': 123@hello
Where this breaks down
regex R: ^(?!.*hello)\w+
string S: hello
The algorithm returns a success because \w+ matches hello, but R will never match a string containing S.
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Grimbal
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Re: Regex and substring
« Reply #4 on: Apr 25th, 2016, 6:40am » |
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We could just check whether S matches R+".+" or ".+"+R.
The only problem I see is with ^ and $ anchors. the ".+" should be inserted after an initial "^" and before a final "$".
PS: oops, possibly correct answer to the wrong question.
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| « Last Edit: Apr 26th, 2016, 7:23am by Grimbal » |
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playful
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Re: Regex and substring
« Reply #5 on: Apr 25th, 2016, 9:45am » |
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on Apr 25th, 2016, 6:40am, Grimbal wrote:| We could just check whether S matches R+".+" or ".+"+R. |
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I think the problem statement is the other way around.
S is a substring of S', not the opposite.
In my example above, with S as "hello", manipulating R as you suggest will never match S.
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