I am going to use regular expressions in a C++ application, but I am not experienced in regex. I want in particular to check a number of strings if they belong to one of the following categories:
X.anystring -> X must be necessarily and exclusively letter (not digit).
XY.anystring -> X, Y must be necessarily and exclusively digits 0-9 (not letters).
How can I check them using regex? What tutorial for regex could you recommend in order to acquaint me with regex?
Seriously, regexp:s are not the right solution for you in this case.
To start with, regexp:s are not part of the C++ language so you will need to use a specific regexp library. (C++11, whoever, include support for regexp:s.)
Secondly, both your use cases can trivially be encoded in plain C++, all you need to do is to loop over the characters in the strings and check if each of them match your requirements.
The current C++11 standard has support for regular expressions, though I'm not sure off the top of my head which compilers support it and are ready to go.
In the meanwhile, the Boost library provides a nice regular expression system for C++ (link here).
In terms of learning about regex, this may help (focuses on using the Boost regex).
An alternative solution that may be simpler for your case would to be just code it yourself. Something like:
bool check_first(const string& myString)
{
if (!isalpha(myString[0]) || myString[1] != '.') return false;
return true;
}
bool check_second(const string& myString)
{
if (!isdigit(myString[0]) || !isdigit(myString[1]) || myString[2] != '.') return false;
return true;
}
X.anystring -> X must be necessarily and exclusively letter (not digit).
Required regex is
[a-zA-Z]\.[\w]+
XY.anystring -> X, Y must be necessarily and exclusively digits 0-9 (not letters).
Required regex is
[0-9]{2}\.[\w]+
Learn more about regexes here. Once you learn about regexes in general, you can apply to any language of your choice.
\b? And the \w in []? (I don't think that \w is actually defined in []; it's normally defined as [^_[:alnum:]], which isn't legal in [].)
letter. \w is indeed allowed inside [] though its not POSIX standard. And \b I agree is redundant in this case. I edited out \b\w is allowed inside [] (and in which version of regular expressions---there are so many). C++11 doesn't seem clear: it's either forbidden (undefined behavior, because not specified), or (my reading, although I'm not at all sure that this was the intention) the equivalent of [[_[:alnum:]]], which, when followed by a +, will match a [, a _ or an alnum, followed by one or more ]. (Given that the OP says "any string", and not a symbol, .* is what is wanted, anyway.)à is a letter, but it won't be matched by [a-zA-Z].If you just want to know if a string matches one or the other, but you don't care which one it matches, you can use:
"(?:(?:[a-zA-Z])|(?:[0-9]{2}))\..*"
Using C++11 regex and ECMAScript syntax.
#include <regex>
std::string str = "OnlyLetter,12345";
std::string x = "([a-z]|[A-Z])+";
std::string y = "[0-9]+";
std::string expression = std::string(x).append(",").append(y);
std::tr1::regex rx(expression);
bool match = std::tr1::regex_match(str.c_str(),rx);
// match = true. Valid String
// match = false. Invalid String. ex.: "OnlyLetter,12s345"
It depends on which regular expression library you're using. But the following should work with both Boost and C++11:
For X.anystring (X is alpha):
"[[:alpha:]]\\..*"
For XY.anystring:
"[[:digit:]][[:digit:]]\\..*"
These are for use with regex_match; if you want to use regex_search,
you'll have to "anchor" the expression to the beginning of the string by
prefixing it with a '^' (but you can drop the final '.*').
