Has anyone in this forum attempted to solve the ACM programming problem http://acm.mipt.ru/judge/problems.pl?browse=yes&problem=024? It is one of the simpler problems in ACM MIPT and the goal is to evaluate an expression consisting of +, -, * and parentheses. Despite the apparent simplicity, I haven't been able to get my solution accepted, apparently because one of the test case expressions has an operator not stated in the problem. I even added support for division ('/') but that too didn't help. Any idea on what other operator needs to be supported? FYI, my program removes all whitespaces from the input before processing so that spaces shouldn't be a problem. Anything not stated in the problem but needs to be taken care of?
You're being bitten by ruby's handling of strings and characters.
curr_ch = #input[i]
gives you an integer, for the input you get, the ASCII code of the character at index i of the input.
curr_ch == '('
for example compares that integer to the string "(", of course that fails. Also the regex matches fail because you pass them an integer where a string is expected.
Replacing all occurrences of some_var = #input[some_index] with some_var = #input[some_index...some_index+1] gives me a programme that seems to work (it works on a few test inputs I gave it). Probably someone who actually knows the quirks of ruby can give you a better fix.
Related
Today I came across the following regular expression and wanted to know what Ruby would do with it:
> "#a" =~ /^[\W].*+$/
=> 0
> "1a" =~ /^[\W].*+$/
=> nil
In this instance, Ruby seems to be ignoring the + character. If that is incorrect, I'm not sure what it is doing with it. I'm guessing it's not being interpreted as a quantifier, since the * is not escaped and is being used as a quantifier. In Perl/Ruby regexes, sometimes when a character (e.g., -) is used in a context in which it cannot be interpreted as a special character, it is treated as a literal. But if that was happening in this case, I would expect the first match to fail, since there is no + in the lvalue string.
Is this a subtly correct use of the + character? Is the above behavior a bug? Am I missing something obvious?
Well, you can certainly use a + after a *. You can read a bit about it on this site. The + after the * is called a possessive quantifier.
What it does? It prevents * from backtracking.
Ordinarily, when you have something like .*c and using this to match abcde, the .* will first match the whole string (abcde) and since the regex cannot match c after the .*, the engine will go back one character at a time to check if there is a match (this is backtracking).
Once it has backtracked to c, you will get the match abc from abcde.
Now, imagine that the engine has to backtrack a few hundred characters, and if you have nested groups and multiple * (or + or the {m,n} form), you can quickly end up with thousands, millions of characters to backtrack, called catastrophic backtracking.
This is where possessive quantifiers come in handy. They actually prevent any form of backtracking. In the above regex I mentioned, abcde will not be matched by .*+c. Once .*+ has consumed the whole string, it cannot backtrack and since there's no c at the end of the string, the match fails.
So, another possible use of possessive quantifiers is that they can improve the performance of some regexes, provided the engine can support it.
For your regex /^[\W].*+$/, I don't think that there's any improvement (maybe a tiny little improvement) that the possessive quantifier provides though. And last, it might easily be rewritten as /^\W.*+$/.
I'm trying to come up with a regex that will elegantly match everything in an URL AFTER the domain name, and before the first ?, the last slash, or the end of the URL, if neither of the 2 exist.
This is what I came up with but it seems to be failing in some cases:
regex = /[http|https]:\/\/.+?\/(.+)[?|\/|]$/
In summary:
http://nytimes.com/2013/07/31/a-new-health-care-approach-dont-hide-the-price/ should return
2013/07/31/a-new-health-care-approach-dont-hide-the-price
http://nytimes.com/2013/07/31/a-new-health-care-approach-dont-hide-the-price?id=2 should return
2013/07/31/a-new-health-care-approach-dont-hide-the-price
http://nytimes.com/2013/07/31/a-new-health-care-approach-dont-hide-the-price should return
2013/07/31/a-new-health-care-approach-dont-hide-the-price
Please don't use Regex for this. Use the URI library:
require 'uri'
str_you_want = URI("http://nytimes.com/2013/07/31/a-new-health-care-approach-dont-hide-the-price").path
Why?
See everything about this famous question for a good discussion of why these kinds of things are a bad idea.
Also, this XKCD really says why:
In short, Regexes are an incredibly powerful tools, but when you're dealing with things that are made from hundred page convoluted standards when there is already a library for doing it faster, easier, and more correctly, why reinvent this wheel?
If lookaheads are allowed
((2[0-9][0-9][0-9].*)(?=\?\w+)|(2[0-9][0-9][0-9].*)(?=/\s+)|(2[0-9][0-9][0-9].*).*\w)
Copy + Paste this in http://regexpal.com/
See here with ruby regex tester: http://rubular.com/r/uoLLvTwkaz
Image using javascript regex, but it works out the same
(?=) is just a a lookahead
I basically set up three matches from 2XXX up to (in this order):
(?=\?\w+) # lookahead for a question mark followed by one or more word characters
(?=/\s+) # lookahead for a slash followed by one or more whitespace characters
.*\w # match up to the last word character
I'm pretty sure that some parentheses were not needed but I just copy pasted.
There are essentially two OR | expressions in the (A|B|C) expression. The order matters since it's like a (ifthen|elseif|else) type deal.
You can probably fix out the prefix, I just assumed that you wanted 2XXX where X is a digit to match.
Also, save the pitchforks everyone, regular expressions are not always the best but it's there for you when you need it.
Also, there is xkcd (https://xkcd.com/208/) for everything:
I am trying to grasp the concept of Regular Expressions but seem to be missing something.
I want to ensure that someone enters a string that ends with .wav in a field. Should be a pretty simple Regular Expression.
I've tried this...
[RegularExpression(#"$.wav")]
but seem to be incorrect. Any help is appreciated. Thanks!
$ is the anchor for the end of the string, so $.wav doesn't make any sense. You can't have any characters after the end of the string. Also, . has a special meaning for regex (it just means 'any character') so you need to escape it.
Try writing
\.wav$
If that doesn't work, try
.*\.wav$
(It depends on if the RegularExpression attribute wants to match the whole string, or just a part of it. .* means 'any character, 0 or more times')
Another thing you should consider is what to do with extra whitespace in the field. Users have a terrible habit of adding extra white space in inputs - its why various .Trim() functions are so important. Here, RegularExpressionAttribute might be evaluated before you can trim the input, so you might want to write this:
.*\.wav[\s]*$
The [\s]* section means 'any whitespace character (tabs, space, linebreak, etc) 0 or more times'.
You should read a tutorial on regex. It's not so hard to understand for simple problems like this. When I was learning I found this site pretty handy: http://www.regular-expressions.info/
can any body tell me how to use regex for negation of string?
I wanna find all line that start with public class and then any thing except first,second and finally any thing else.
for example in the result i expect to see public class base but not public class myfirst:base
can any body help me please??
Use a negative lookahead:
public\s+class\s+(?!first|second).+
If Peter is correct and you're using Visual Studio's Find feature, this should work:
^:b*public:b+class:b+~(first|second):i.*$
:b matches a space or tab
~(...) is how VS does a negative lookahead
:i matches a C/C++ identifier
The rest is standard regex syntax:
^ for beginning of line
$ for end of line
. for any character
* for zero or more
+ for one or more
| for alternation
Both the other two answers come close, but probably fail for different reasons.
public\s+class\s+(?:(?!first|second).)+
Note how there is a (non-capturing) group around the negative lookahead, to ensure it applies to more than just the first position.
And that group is less restrictive - since . excludes newline, it's using that instead of \S, and the $ is not necessary - this will exclude the specified words and match others.
No slashes wrapping the expression since those aren't required in everything and may confuse people that have only encountered string-based regex use.
If this still fails, post the exact content that is wrongly matched or missed, and what language/ide you are using.
Update:
Turns out you're using Visual Studio, which has it's own special regex implementation, for some unfathomable reason. So, you'll be wanting to try this instead:
public:b+class:b+~(first|second)+$
I have no way of testing that - if it doesn't work, try dropping the $, but otherwise you'll have to find a VS user. Or better still, the VS engineer(s) responsible for this stupid non-standard regex.
Here is something that should work for you
/public\sclass\s(?:[^fs\s]+|(?!first|second)\S)+(?=\s|$)/
The second look a head could be changed to a $(end of line) or another anchor that works for your particular use case, like maybe a '{'
Edit: Try changing the last part to:
(?=\s|$)
Many programming languages allow trailing commas in their grammar following the last item in a list. Supposedly this was done to simplify automatic code generation, which is understandable.
As an example, the following is a perfectly legal array initialization in Java (JLS 10.6 Array Initializers):
int[] a = { 1, 2, 3, };
I'm curious if anyone knows which language was first to allow trailing commas such as these. Apparently C had it as far back as 1985.
Also, if anybody knows other grammar "peculiarities" of modern programming languages, I'd be very interested in hearing about those also. I read that Perl and Python for example are even more liberal in allowing trailing commas in other parts of their grammar.
I'm not an expert on the commas, but I know that standard Pascal was very persnickity about semi-colons being statement separators, not terminators. That meant you had to be very very careful about where you put one if you didn't want to get yelled at by the compiler.
Later Pascal-esque languages (C, Modula-2, Ada, etc.) had their standards written to accept the odd extra semicolon without behaving like you'd just peed in the cake mix.
I just found out that a g77 Fortran compiler has the -fugly-comma Ugly Null Arguments flag, though it's a bit different (and as the name implies, rather ugly).
The -fugly-comma option enables use of a single trailing comma to mean “pass an extra trailing null argument” in a list of actual arguments to an external procedure, and use of an empty list of arguments to such a procedure to mean “pass a single null argument”.
For example, CALL FOO(,) means “pass two null arguments”, rather than “pass one null argument”. Also, CALL BAR() means “pass one null argument”.
I'm not sure which version of the language this first appeared in, though.
[Does anybody know] other grammar "peculiarities" of modern programming languages?
One of my favorites, Modula-3, was designed in 1990 with Niklaus Wirth's blessing as the then-latest language in the "Pascal family". Does anyone else remember those awful fights about where semicolon should be a separator or a terminator? In Modula-3, the choice is yours! The EBNF for a sequence of statements is
stmt ::= BEGIN [stmt {; stmt} [;]] END
Similarly, when writing alternatives in a CASE statement, Modula-3 let you use the vertical bar | as either a separator or a prefix. So you could write
CASE c OF
| 'a', 'e', 'i', 'o', 'u' => RETURN Char.Vowel
| 'y' => RETURN Char.Semivowel
ELSE RETURN Char.Consonant
END
or you could leave off the initial bar, perhaps because you prefer to write OF in that position.
I think what I liked as much as the design itself was the designers' awareness that there was a religious war going on and their persistence in finding a way to support both sides.
Let the programmer choose!
P.S. Objective Caml allows permissive use of | in case expressions whereas the earlier and closely related dialect Standard ML does not. As a result, case expressions are often uglier in Standard ML code.
EDIT: After seeing T.E.D.'s answer I checked the Modula-2 grammar and he's correct, Modula-2 also supported semicolon as terminator, but through the device of the empty statement, which makes stuff like
x := x + 1;;;;;; RETURN x
legal. I suppose that's not a bad thing. Modula-2 didn't allow flexible use of the case separator |, however; that seems to have originated with Modula-3.
Something which has always galled me about C is that although it allows an extra trailing comma in an intializer list, it does not allow an extra trailing comma in an enumerator list (for defining the literals of an enumeration type). This little inconsistency has bitten me in the ass more times than I care to admit. And for no reason!