CODEWITHSUNDEEP
regexc++11
tonso
tonso

Reputation: 1768

C++ regex: replace a pattern in multiple lines

I'd like to remove certain lines (let's assume 'aaa' and 'bbb') from a multiline string with the standard C++ regex library. With ECMAScript replace syntax, I would use the next pattern: /^(aaa|bbb)$/gm. I tried to pass something like ^(aaa|bbb)$ as regex argument to std::regex_replace, but it's not working, as ^ and $ seem to not get matched as the beginning and the end of line, though I didn't pass match_not_eol or match_not_bol as match flags. So how the C++ pattern should look like for this case?

Upvotes: 2

Views: 1814

Answers (1)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 627103

The regex_replace reference does not list any flag to redefine the ^ and $ meanings. Thus, when you need to search and replace lines, use the capturing group (^|\r?\n) before and a lookahead (?=\r?\n|$) after the main pattern (and replace with backreference $1). Surely, the patterns can be adjusted to allow or not empty lines after the replacement.

See IDEONE demo:

std::string s("ccc\r\naaa\r\ngggg");
std::regex ee(R"((^|\r?\n)(?:aaa|bbb)(?=\r?\n|$))");
std::string newtext = std::regex_replace( s, ee, "$1" );
std::cout << newtext << std::endl;

Output:

ccc

gggg

Upvotes: 1

Related Questions