use std::regex to find contents of a function - c++

Question

so let's say I have a main function with some arbitrary code:
void main(){
//Some random code
int a = 5;
int b = a + 7;
}
and the text of this function is stored inside an std::string:
std::string mystring("void main(){ //Some random code int a = 5; int b = a + 7;}");
I want to use std::regex in order to extract out the body of the function. So the result I would be getting back is:
"//Some random code int a= 5; int b = a + 7;"
My issue is I do not know how to format the regular expression to get what I want. Here is my code I have right now:
std::string text("void main(){ //Some random code int a = 5; int b = a + 7;}");
std::regex expr ("void main()\\{(.*?)\\}");
std::smatch matches;
if (std::regex_match(text, matches, expr)) {
for (int i = 1; i < matches.size(); i++) {
std::string match (matches[i].first, matches[i].second);
std::cout << "matches[" << i << "] = " << match << std::endl;
}
}
My regex is completely off and returns no matches. What do I need to make my regex in order for this to work?

Answer 1

As discussed in the comments OP only wants to "extract the text inside the function body, regardless of what that text is".
#OP:
Your regex is wrong as you don't escape the parenthesis for main().
Changing the regex to "void main\\(\\)\\{(.*?)\\}" will work.
I also recommend to use size_t for i in your for-loop so you don't compare signed with unsigned (std::smatch::size() returns size_t).
#include <iostream>
#include <regex>
int main()
{
std::string text("void main(){ //Some random code int a = 5; int b = a + 7;}");
std::regex expr("void main\\(\\)\\{(.*?)\\}");
std::smatch matches;
if (std::regex_match(text, matches, expr)) {
for (size_t i = 1; i < matches.size(); i++) {
std::string match(matches[i].first, matches[i].second);
std::cout << "matches[" << i << "] = " << match << std::endl;
}
}
}
Output:
matches[1] = //Some random code int a = 5; int b = a + 7;
This solution fails for the input "void main(){ while(true){ //Some random code int a = 5; int b = a + 7; } }"
The easiest solution to this would be to change the regex to "^void main\\(\\)\\{(.*?)\\}$" but that requires the input to start with "void main(){" and end with "}"
As proposed by Revolver_Ocelot you can also add some whitespace matching into the regex to make it a little bit more flexible.

Answer 2

As suggested in your use case it would probably be the best to just rely on string search and matching of braces.
#include <iostream>
#include <regex>
std::string getBody(const std::string& functionDef, const std::string& text)
{
size_t pos = 0;
do
{
if ((pos = text.find(functionDef, pos)) == std::string::npos)
continue;
pos += functionDef.length();
size_t firstSemicolon = text.find(";", pos);
size_t firstOpen = text.find("{", pos);
size_t firstClose = text.find("}", pos);
if (firstSemicolon != std::string::npos && firstSemicolon < firstOpen) //Only function declaration
continue;
if (firstOpen == std::string::npos || firstClose == std::string::npos || firstClose < firstOpen) //Mismatch
continue;
size_t bodyStart = pos = firstOpen + 1;
size_t bracesCount = 1;
do
{
firstOpen = text.find("{", pos);
firstClose = text.find("}", pos);
if (firstOpen == std::string::npos && firstClose == std::string::npos)//Mismatch
{
pos = std::string::npos;
continue;
}
//npos is always larger
if (firstOpen < firstClose)
{
bracesCount++;
pos = firstOpen + 1;
}
else if (firstOpen > firstClose)
{
bracesCount--;
if (bracesCount == 0)
{
size_t bodySize = firstClose - bodyStart;
return text.substr(bodyStart, bodySize);
}
pos = firstClose + 1;
}
else
{
//Something went terribly wrong...
pos = std::string::npos;
continue;
}
} while (pos != std::string::npos);
}
while (pos != std::string::npos);
return std::string();
}
int main()
{
std::string text("void main(); int test(); void main(){ while(true){ //Some {random} code int a = 5; int b = a + 7; } } int test(){ return hello; } ");
std::cout << getBody("void main()", text) << std::endl;
std::cout << getBody("int test()", text) << std::endl;
}
Output:
while(true){ //Some {random} code int a = 5; int b = a + 7; }
return hello;
The code can also handle newlines and skips function declarations. I tried to write it as clear as possible.
If there are still questions feel free to ask.