smatch | Code to match points on the sphere using the healpix scheme | Game Engine library

As regex_replace creates a copy of your string you could simply compare the replaced string with the original one and only "store" the new one if they differ.

For C++14 it seems that regex_replace returns a pointer to the last place it has written to:

https://www.cplusplus.com/reference/regex/regex_replace/ Versions 5 and 6 return an iterator that points to the element past the last character written to the sequence pointed by out.

It's because the iterator types do not match, if you look at the source of std::smatch in Visual Studio you'll find this.

According to the C++ ECMAScript regex flavor reference,

The decimal escape \0 is NOT a backreference: it is a character escape that represents the nul character. It cannot be followed by a decimal digit.

So, to match a NULL char, you need to use \0 literal text, a literal \ char and a 0 char. You can define it with a regular string literal as "\\0" or - better - with a raw string literal, R"(\0)".

The following prints "Success":

What you are looking for is the position() function of match_results, so something like that should work:

\ is used as a escape sequence in C++. You have to write \ as \\ to pass it to regex engine.

In the first place, is good for you to keep in mind that the end of a string is not the next . character, it's the end of memory region that the variable subject refers to. So, when you match the end of the string, the regex engine will go to the end of that memory.

For instance, we can have string str = "......................."; and the end of the string will be the last . character.

What you are trying to do, I suppose, is match the word "link" until the next .. For this, you should define a charset (consisting of upper and lowercase letters, numbers, spaces, and colon characters, according to your testcases).

A regex resembling this one link([0-9a-z ]*)\. can be used.

Also, before you use those, I suggest you test your regexes in some places, like RegExr.

You need to check each return value of each of the str.find() calls like I do for the first one but this is the gist of it. Might want to just search for the tag, then the id, but then you also need to check for non-existing id for for that tag:

Your online regex test is wrong because your actual text is {"|1|":"A","|2|":"B","|37|":"4234235","|4|":"C"}, you may see that your regex does not match it.

Besides, you are using an ECMAScript regex flavor in std::regex, but your regex is PCRE compliant. E.g. ECMAScript regex does not support \K match reset operator.

You need a "\|37\|":"(\d+) regex, see the regex demo. Details:

• "\|37\|":" - literal "|37|":" text
• (\d+) - Group 1: one or more digits.

See the C++ demo:

According to the C++17 Standard:

28.10 Class template match_results [ re.results ]

4 The sub_match object stored at index 0 represents sub-expression 0, i.e., the whole match. In this case the sub_match member matched is always true. The sub_match object stored at index n denotes what matched the marked sub-expression n within the matched expression. If the sub-expression n participated in a regular expression match then the sub_match member matched evaluates to true, and members first and second denote the range of characters [first,second) which formed that match. Otherwise matched is false, and members first and second point to the end of the sequence that was searched.

[ Note: The sub_match objects representing different sub-expressions that did not participate in a regular expression match need not be distinct. — end note ]

Now m.position(n) returns (*this)[n].first.

Given that "[If] matched is false, [then] members first and second point to the end of the sequence that was searched" ...

This means m.position(n) should point "to the end of the sequence that was searched".