I don't understand how to retrieve all groups using regexp in c++
An example:
const std::string s = "1,2,3,5";
std::regex lrx("^(\\d+)(,(\\d+))*$");
std::smatch match;
if (std::regex_search(s, match, lrx))
{
int i = 0;
for (auto m : match)
std::cout << " submatch " << i++ << ": "<< m << std::endl;
}
Gives me the result
submatch 0: 1,2,3,5
submatch 1: 1
submatch 2: ,5
submatch 3: 5
I am missing 2 and 3
You cannot use the current approach, since std::regex does not allow storing of the captured values in memory, each time a part of the string is captured, the former value in the group is re-written with the new one, and only the last value captured is available after a match is found and returned. And since you defined 3 capturing groups in the pattern, you have 3+1 groups in the output.
Mind also, that std::regex_search only returns one match, while you will need multiple matches here.
So, what you may do is to perform 2 steps: 1) validate the string using the pattern you have (no capturing is necessary here), 2) extract the digits (or split with a comma, that depends on the requirements).
A C++ demo:
#include <string>
#include <iostream>
#include <regex>
using namespace std;
int main() {
std::regex rx_extract("[0-9]+");
std::regex rx_validate(R"(^\d+(?:,\d+)*$)");
std::string s = "1,2,3,5";
if (regex_match(s, rx_validate)) {
for(std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), rx_extract);
i != std::sregex_iterator();
++i)
{
std::smatch m = *i;
std::cout << m.str() << '\n';
}
}
return 0;
}
Output:
1
2
3
5
Related
I'm looking for a regex pattern that returns true if found 7 numbers on given string. There's no order so if a string is set to: "100 my, str1ng y000" it catches that.
RegEx alone won't count exact occurrences for you, it would return true even if there are more than 7 digits in the string because it would try to find out at least 7 digits in the string.
You can use below code to test exact number (7 in your case) of digits in any string:
var temp = "100 my, str1ng y000 3c43fdgd";
var count = (temp.match(/\d/g) || []).length;
alert(count == 7);
I will show you an C++ Example that
Shows a regex for extracting digit groups
Shows a regex for matching at least 7 digits
Shows, if there is a match for the requested predicate
Shows the number of digits in the string (no regex needed)
Shows the group of digits
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <regex>
// Our test data (raw string). So, containing also \" and so on
std::string testData("100 my, str1ng y000");
std::regex re1(R"#((\d+))#"); // For extracting digit groups
std::regex re2(R"#((\d.*){7,})#"); // For regex match
int main(void)
{
// Define the variable id as vector of string and use the range constructor to read the test data and tokenize it
std::vector<std::string> id{ std::sregex_token_iterator(testData.begin(), testData.end(), re1, 1), std::sregex_token_iterator() };
// Match the regex. Should have at least 7 digits somewhere
std::smatch base_match;
bool containsAtLeast7Digits = std::regex_match(testData, base_match, re2);
// Show result on screen
std::cout << "\nEvaluating string '" << testData <<
"'\n\nThe predicate 'contains-at-leats-7-digits' is " << std::boolalpha << containsAtLeast7Digits <<
"\n\nIt contains overall " <<
std::count_if(
testData.begin(),
testData.end(),
[](const char c) {
return std::isdigit(static_cast<int>(c));
}
) << " digits and " << id.size() << " digit groups. These are:\n\n";
// Print complete vector to std::cout
std::copy(id.begin(), id.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
return 0;
}
Please note: Use std::count for counting. Faster and easier.
Hope this helps . . .
Very new to the c++ regex libraries.
We are trying to parse a line
*10 abc
We want to parse/split this line into only two tokens:
10
abc
I have tried multiple things such as regex_search but I do get 3 matches. First match is whole match and second, third are sub sequences matches. My question would be that
How can we get only two matches(10 & abc) from above string. Snapshot of what I have tried:
#include <regex>
#include <iostream>
int main() {
const std::string t = "*10 abc";
std::regex rgxx("\\*(\\d+)\\s+(.+)");
std::smatch match;
bool matched1 = std::regex_search(t.begin(), t.end(), match, rgxx);
std::cout << "Matched size " << match.size() << std::endl;
for(int i = 0 ; i < match.size(); ++i) {
std::cout << i << " match " << match[i] << std::endl;
}
}
Output:
Matched size 3
0 match *10 abc
1 match 10
2 match abc
0 match is the one which I do not want.
I am open to use boost libraries/regexes as well. Thank you.
There is nothing really wrong with your code per se. The zero match is just the entire string, which matched the regex pattern. If you only want the two captured terms, then just print the first and second capture groups:
const std::string t = "*10 abc";
std::regex rgxx("(\\d+)\\s+(.+)");
std::smatch match;
bool matched1 = std::regex_search(t.begin(), t.end(), match, rgxx);
std::cout << "Matched size " << match.size() << std::endl;
for (int i=1; i < match.size(); ++i) {
std::cout << i << " match " << match[i] << std::endl;
}
Matched size 3
1 match 10
2 match abc
So, the lesson here is that the first entry in the match array (index of zero) will always be the entire string.
I'm a bit confused about the following C++11 code:
#include <iostream>
#include <string>
#include <regex>
int main()
{
std::string haystack("abcdefabcghiabc");
std::regex needle("abc");
std::smatch matches;
std::regex_search(haystack, matches, needle);
std::cout << matches.size() << std::endl;
}
I'd expect it to print out 3 but instead I get 1. Am I missing something?
You get 1 because regex_search returns only 1 match, and size() will return the number of capture groups + the whole match value.
Your matches is...:
Object of a match_results type (such as cmatch or smatch) that is filled by this function with information about the match results and any submatches found.
If [the regex search is] successful, it is not empty and contains a series of sub_match objects: the first sub_match element corresponds to the entire match, and, if the regex expression contained sub-expressions to be matched (i.e., parentheses-delimited groups), their corresponding sub-matches are stored as successive sub_match elements in the match_results object.
Here is a code that will find multiple matches:
#include <string>
#include <iostream>
#include <regex>
using namespace std;
int main() {
string str("abcdefabcghiabc");
int i = 0;
regex rgx1("abc");
smatch smtch;
while (regex_search(str, smtch, rgx1)) {
std::cout << i << ": " << smtch[0] << std::endl;
i += 1;
str = smtch.suffix().str();
}
return 0;
}
See IDEONE demo returning abc 3 times.
As this method destroys the input string, here is another alternative based on the std::sregex_iterator (std::wsregex_iterator should be used when your subject is an std::wstring object):
int main() {
std::regex r("ab(c)");
std::string s = "abcdefabcghiabc";
for(std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
i != std::sregex_iterator();
++i)
{
std::smatch m = *i;
std::cout << "Match value: " << m.str() << " at Position " << m.position() << '\n';
std::cout << " Capture: " << m[1].str() << " at Position " << m.position(1) << '\n';
}
return 0;
}
See IDEONE demo, returning
Match value: abc at Position 0
Capture: c at Position 2
Match value: abc at Position 6
Capture: c at Position 8
Match value: abc at Position 12
Capture: c at Position 14
What you're missing is that matches is populated with one entry for each capture group (including the entire matched substring as the 0th capture).
If you write
std::regex needle("a(b)c");
then you'll get matches.size()==2, with matches[0]=="abc", and matches[1]=="b".
EDIT: Some people have downvoted this answer. That may be for a variety of reasons, but if it is because it does not apply to the answer I criticized (no one left a comment to explain the decision), they should take note that W. Stribizew changed the code two months after I wrote this, and I was unaware of it until today, 2021-01-18. The rest of the answer is unchanged from when I first wrote it.
#stribizhev's solution has quadratic worst case complexity for sane regular expressions. For insane ones (e.g. "y*"), it doesn't terminate. In some applications, these issues could be DoS attacks waiting to happen. Here's a fixed version:
string str("abcdefabcghiabc");
int i = 0;
regex rgx1("abc");
smatch smtch;
auto beg = str.cbegin();
while (regex_search(beg, str.cend(), smtch, rgx1)) {
std::cout << i << ": " << smtch[0] << std::endl;
i += 1;
if ( smtch.length(0) > 0 )
std::advance(beg, smtch.length(0));
else if ( beg != str.cend() )
++beg;
else
break;
}
According to my personal preference, this will find n+1 matches of an empty regex in a string of length n. You might also just exit the loop after an empty match.
If you want to compare the performance for a string with millions of matches, add the following lines after the definition of str (and don't forget to turn on optimizations), once for each version:
for (int j = 0; j < 20; ++j)
str = str + str;
I need to get all regex matches and their positions.
For example, I have this regex:
std::regex r("(a)|(b)|(c)");
And this input text:
std::string text("abcab");
Now I want to loop the matches there in every loop I can access all occurrences from one match. So in first loop I could get "a" at position 0 and "a" at position 3. In second loop it'd be "b" at 1 and "b" at 4. And in third loop it'd be "c" at position 2. How can I do this?
Currently I have every regex part separately (regex for (a), (b) and (c)) and go through them one by one. But there are quite many of them so I'm looking for better/faster solution.
You can declare string vectors to store the captured values in, and then check which alternative branch matched, and add it to the corresponding vector.
Here is a C++ demo:
#include <string>
#include <iostream>
#include <regex>
using namespace std;
int main() {
std::regex r("(a)|(b)|(c)");
std::string s = "abcab";
std::vector<std::string> astrings; // Declare the vectors to
std::vector<std::string> bstrings; // populate with the contents
std::vector<std::string> cstrings; // of capturing groups
for(std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
i != std::sregex_iterator();
++i)
{
std::smatch m = *i;
if (m[1].matched) { // Check if Group 1 matched and
astrings.push_back(m[1].str()); // Put a value into a string vector
}
else if (m[2].matched) { // Check if Group 2 matched and
bstrings.push_back(m[2].str()); // Put a value into b string vector
}
else if (m[3].matched) { // Check if Group 3 matched and
cstrings.push_back(m[3].str()); // Put a value into c string vector
}
}
// Printing vectors - DEMO
for (auto i: astrings)
std::cout << i << ' ';
std::cout << "\n";
for (auto i: bstrings)
std::cout << i << ' ';
std::cout << "\n";
for (auto i: cstrings)
std::cout << i << ' ';
return 0;
}
You may also consider using std::regex_constants::optimize flag when declaring the regexp (see Galik's comment).
Ok, so I'm working with C++ regex and I'm not quite sure how to go about extracting the numbers that I want from my expression.
I'm building an expression BASED on numbers, but not sure how to pull them back out.
Here's my string:
+10.7% Is My String +5 And Some Extra Stuff Here
I use that string to pull the numbers
10 , 7 , 5 out and add them to a vector, no big deal.
I then change that string to become a regex expression.
\+([0-9]+)\.([0-9]+)% Is My String \+([0-9]+) And Some Extra Stuff Here
Now how do I go about using that regexp expression to MATCH my starting string and extracting the numbers back out.
Something along the lines of using the match table?
You must iterate over the submatches to extract them.
Example:
#include <iostream>
#include <string>
#include <regex>
int main()
{
std::string input = "+10.7% Is My String +5 And Some Extra Stuff Here";
std::regex rx("\\+([0-9]+)\\.([0-9]+)% Is My String \\+([0-9]+) And Some Extra Stuff Here");
std::smatch match;
if (std::regex_match(input, match, rx))
{
for (std::size_t i = 0; i < match.size(); ++i)
{
std::ssub_match sub_match = match[i];
std::string num = sub_match.str();
std::cout << " submatch " << i << ": " << num << std::endl;
}
}
}
Output:
submatch 0: +10.7% Is My String +5 And Some Extra Stuff Here
submatch 1: 10
submatch 2: 7
submatch 3: 5
live example: https://ideone.com/01XJDF