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TCL: Can Anyone Explain ?: in regular expression
I am getting confusion between ? and ?: .
? means preceding character may or may not be present.
Then I am not understanding what (?:) indicates.
Can Anyone please Explain this.
([0-9]+(?:\.[0-9]*)?)
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It indicates that the subpattern is a non-capture subpattern. That means whatever is matched in (?:\w+\s) , even though it's enclosed by () it won't appear in the list of matches, only (\w+) will.
We say that two regular expressions R and S are equivalent if they describe the same language. In other words, if L(R) = L(S) for two regular expressions R and S then R = S.
In short regular expressions does not allow the pattern to refer to itself. You cannot say: at this point in the syntax match the whole pattern again. To put it another way, regular expressions only matches linearly, it does not contain a stack which would allow it to keep track of how deep it is an a nested pattern.
Suppose, you were trying to look for something like ABC123 or ABC123.45 in your input String and you wanted to capture the letters and the numbers separately. You would use a regex (a bit similar to yours) like
([A-Z]+)([0-9]+(\.[0-9]+)?)
The above regex would match ABC123.45 and provide three groups as well that represent sub-parts of the whole match and are decided by where you put those () brackets. So, given our regex (without using ?:) we got
Group 1 = ABC
Group 2 = 123.45
Group 3 = .45
Now, it may not make much sense to capture the decimal portion always and it actually has already been captured in our Group 2 as well. So, how would you make that group () non capturing? Yes, by using ?: at the start as
([A-Z]+)([0-9]+(?:\.[0-9]+)?)
Now, you only get the two desired groups
Group 1 = ABC
Group 2 = 123.45
Notice, I also changed the last part of the regex from \.[0-9]* to \.[0-9]+. This would prevent a match on 123. i.e. numbers without a decimal part but still having a dot.
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