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[\-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([\-a-zA-Z0-9()@:%_+.~#?&//=]*)
I used this concept once to write the validation logic for an "IP RegEx filter" setting. The goal was to let users configure an IP filter using RegEx (no, marketing people don't get CIDRs, and they knew RegEx's from Google Analytics). How could I define a valid RegEx for this? The intersection with the RegEx of "all IPv4 addresses" is not empty, and not equal to the RegEx of "all IPv4 addresses". Prevented many compl…
I used this concept once to write the validation logic for an "IP RegEx filter" setting. The goal was to let users configure an IP filter using RegEx (no, marketing people don't get CIDRs, and they knew RegEx's from Google Analytics). How could I define a valid RegEx for this? The intersection with the RegEx of "all IPv4 addresses" is not empty, and not equal to the RegEx of "all IPv4 addresses". Prevented many compl…
Wouldn't a simpler solution work here? Instead of trying to validate the filter regex, show some sample IP addresses or let the user insert a set of addresses, and then show which ones the filter matches and which ones it doesn't. Also helps address the problem of incorrect filters.
The amazing page computes binary relations between pairs of regular expressions and shows a graphical representation of the DFA. It’s a really incredible demonstration of some highly non-trivial operations on regular expressions.
It's very cool, but also no wonder that it doesn't support all those features of regexes which technically make them not regular expressions anymore. Though, I would have thought ^ and $ anchors shouldn't be a problem?
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Wouldn't a simpler solution work here? Instead of trying to validate the filter regex, show some sample IP addresses or let the user insert a set of addresses, and then show which ones the filter matches and which ones it doesn't. Also helps address the problem of incorrect filters.
The odds of the sample addresses matching is essentially zero, and adding work to the user is counterproductive.
I wanted to see the intersection between syntactically valid URLs and email addresses, but just entering the URL regex (cf. below) already takes too long to process for the page. [\-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([\-a-zA-Z0-9()@:%_+.~#?&//=]*) (source: https://stackoverflow.com/a/3809435/623763 )
if you replace that with (...)+ then it seems to work (at least for me). smaller expressions like (...){1,6} should be fine.
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The odds of the sample addresses matching is essentially zero, and adding work to the user is counterproductive.
I'm not sure I agree — most common regex editing tools available online include a section for adding test strings to verify what you actually wrote is correct. Clearly there is a benefit to it. In similar vein, allowing the user to test before they commit and then test actually reduces their work load, they don't have to drop and then reload the whole regex in their mind.
Kinda related but I'm looking for something that could give me the number of possible matching strings for a simple regex. Does such a tool exist ?
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I think that's just the ordinal corresponding to lexicographic order on the words in the language, so yeah that should work. I wonder how easy it is to calculate...
normally you would use an ordinal number [1] to label individual elements of an infinite set while using a cardinal number [2] to measure the size of the set. i believe the cardinality of a set of words from a finite alphabet (with more than one member) is equivalent to the cardinality of the real numbers. this means that the cardinality of .* is c. unfortunately, i don't think that cardinality gets us very far when…
As you've rightly noted the latter equivalence class gets us nothing so throwing away the ordering is a bit of a waste. Of all mathematical concepts 'size' is easily the most subjective so picking one that is interesting is better than trying to be 'correct'.
In particular a*b* is exactly equivalent to ω^2, since a^n b^m < a^x b^y iff n < x or n=x and m<y. This gives an order preserving isomorphism between words of the form a^n b^m and tuples (n,m) with lexicographic ordering.