How Can I Use a Regex on the Reverse of a string?

Question

I want to use a regex on the reverse of a string.

I can do the following but all my sub_matches are reversed:

string foo("lorem ipsum"); match_results<string::reverse_iterator> sm; if (regex_match(foo.rbegin(), foo.rend(), sm, regex("(\\w+)\\s+(\\w+)"))) { cout << sm[1] << ' ' << sm[2] << endl; } else { cout << "bad\n"; }

[Live example]

What I want is to get out:

ipsum lorem

Is there any provision for getting the sub-matches that are not reversed? That is, any provision beyond reversing the strings after they're matched like this:

string first(sm[1]); string second(sm[2]); reverse(first.begin(), first.end()); reverse(second.begin(), second.end()); cout << first << ' ' << second << endl;

EDIT:

It has been suggested that I update the question to clarify what I want:

Running the regex backwards on the string is not about reversing the order that the matches are found in. The regex is far more complex that would be valuable to post here, but running it backwards saves me from needing a look ahead. This question is about the handling of sub-matches obtained from a match_results<string::reverse_iterator>. I need to be able to get them out as they were in the input, here foo. I don't want to have to construct a temporary string and run reverse on it for each sub-match. How can I avoid doing this.

So you want to find a match, then reverse the word order of the matching fragments, then write it to cout/wherever? — Michael McPherson
– Michael McPherson, Commented Aug 4, 2015 at 17:03
@MichaelMcPherson Well I want to run the regex on the reverse of the string, yes. But I don't want my matches to be reversed... if that's possible. — Jonathan Mee
– Jonathan Mee, Commented Aug 4, 2015 at 17:15
Okay, so you want to run a regex on a string that has been reversed character for character, and then output regular (non-reversed) words? — Michael McPherson
– Michael McPherson, Commented Aug 4, 2015 at 17:17
Either way, this seems to be a bound up problem. If there was more information about the regex or the target string, maybe people could give you a better answer. But for the data presented here ("A string has been reversed. I have n matches in reversed form. How to I un-reverse them?") I think running reverse is the only actual answer. — Michael McPherson
– Michael McPherson, Commented Aug 4, 2015 at 17:59
If you are running reverse iterators on the input string you are going to end up with reversed string matches. The way I see it you have two options. a) reverse the matches. b) use reverse iterators from the matches in whatever it is you want to do with them. — Galik
– Galik, Commented Aug 4, 2015 at 18:07

Galik · Accepted Answer · 2015-08-04 17:08:08Z

5

You could just reverse the order in which you use the results:

#include <regex> #include <string> #include <iostream> using namespace std; int main() { string foo("lorem ipsum"); smatch sm; if (regex_match(foo, sm, regex("(\\w+)\\s+(\\w+)"))) { cout << sm[2] << ' ' << sm[1] << endl; // use second as first } else { cout << "bad\n"; } }

Output:

ipsum lorem

answered Aug 4, 2015 at 17:08

Galik

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2 Comments

Jonathan Mee Over a year ago

This does not accomplish what I want. The regex is significantly more complex than the one that I have posted here in the example. In fact there is no way to accomplish this regex without look ahead support in the regex library. I am able to get around some of the complexity by iterating backwards, but I do not want to have to then construct strings and reverse them for each sub-match.

Galik Over a year ago

@JonathanMee Well maybe you could try posting the actual regex? Or at least something that possesses the same intractability as your problem?

Community · Accepted Answer · 2017-05-23 11:58:47Z

This is absolutely possible! The key is in the fact that a sub_match inherits from pair<BidirIt, BidirIt>. Since sub_matches will be obtained from: match_results<string::reverse_iterator> sm, the elements of the pair a sub_match inherits from will be string::reverse_iterators.

So for any given sub_match from sm you can get the forward range from it's second.base() to it's first.base(). You don't have to construct strings to stream ranges but you will need to construct an ostream_iterator:

ostream_iterator<char> output(cout); copy(sm[1].second.base(), sm[1].first.base(), output); output = ' '; copy(sm[2].second.base(), sm[2].first.base(), output);

Take heart though, there is a better solution on the horizon! This answer discusses string_literals as of right now no action has been taken on them, but they have made it into the "Evolution Subgroup".

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