{"id":5745,"date":"2019-07-18T05:59:58","date_gmt":"2019-07-18T05:59:58","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/more-rules-to-the-regular-expression-library\/"},"modified":"2023-06-26T10:04:51","modified_gmt":"2023-06-26T10:04:51","slug":"more-rules-to-the-regular-expression-library","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/more-rules-to-the-regular-expression-library\/","title":{"rendered":"More Rules about the Regular Expression Library"},"content":{"rendered":"

There is more to write about the usage of regular expressions than I wrote in my last post The Regular Expression Library<\/a>. Let’s continue.<\/p>\n

<\/p>\n

 <\/p>\n

\"antique<\/p>\n

 <\/h2>\n

The text determines the regular expression, the result, and the capture groups<\/h2>\n

First of all, the type of text determines the character type of the regular expression, the type of the search result, and the type of the capture group. Of course, my argument also holds if other parts of the regex machinery are applied to text. Okay, that sounds worse than it is. Capture is a subexpression in your search result, which you can define in round braces. I wrote already about it in my last post The Regular Expression Library<\/a>.<\/p>\n

The table gives all the types depending on the text type.<\/p>\n

\"CCoreGuidelinesMoreRulesToRegexNew\"<\/p>\n

 <\/p>\n

Here is an example of all the variations of std::regex_search <\/span>depending on the text type.<\/p>\n

 <\/div>\n
\n
\/\/ search.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n#include <regex><\/span>\r\n#include <string><\/span>\r\n\r\nint<\/span> main<\/span>(){\r\n\r\n  std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n  \/\/ regular expression for time<\/span>\r\n  std::<\/span>regex crgx(\"([01]?[0-9]|2[0-3]):[0-5][0-9]\"<\/span>);\r\n\r\n  \/\/ const char*<\/span>\r\n  std::<\/span>cout <<<\/span> \"const char*\"<\/span> <<<\/span> std::<\/span>endl;\r\n  std::<\/span>cmatch cmatch;\r\n\r\n  const<\/span> char<\/span>*<\/span> ctime{\"Now it is 23:10.\"<\/span>};\r\n\r\n  if<\/span> (std::<\/span>regex_search(ctime, cmatch, crgx)){\r\n\r\n     std::<\/span>cout <<<\/span> ctime <<<\/span> std::<\/span>endl;\r\n     std::<\/span>cout <<<\/span> \"Time: \"<\/span> <<<\/span> cmatch[0<\/span>] <<<\/span> std::<\/span>endl;\r\n\r\n   }\r\n\r\n  std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n  \/\/ std::string<\/span>\r\n  std::<\/span>cout <<<\/span> \"std::string\"<\/span> <<<\/span> std::<\/span>endl;\r\n  std::<\/span>smatch smatch;\r\n\r\n  std::<\/span>string stime{\"Now it is 23:25.\"<\/span>};\r\n  if<\/span> (std::<\/span>regex_search(stime, smatch, crgx)){\r\n\r\n    std::<\/span>cout <<<\/span> stime <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"Time: \"<\/span> <<<\/span> smatch[0<\/span>] <<<\/span> std::<\/span>endl;\r\n\r\n  }\r\n\r\n  std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n  \/\/ regular expression holder for time<\/span>\r\n  std::<\/span>wregex wrgx(L\"([01]?[0-9]|2[0-3]):[0-5][0-9]\"<\/span>);\r\n\r\n  \/\/ const wchar_t*<\/span>\r\n  std::<\/span>cout <<<\/span> \"const wchar_t* \"<\/span> <<<\/span> std::<\/span>endl;\r\n  std::<\/span>wcmatch wcmatch;\r\n\r\n  const<\/span> wchar_t<\/span>*<\/span> wctime{L\"Now it is 23:47.\"<\/span>};\r\n\r\n  if<\/span> (std::<\/span>regex_search(wctime, wcmatch, wrgx)){\r\n\r\n       std::<\/span>wcout <<<\/span> wctime <<<\/span> std::<\/span>endl;\r\n       std::<\/span>wcout <<<\/span> \"Time: \"<\/span> <<<\/span> wcmatch[0<\/span>] <<<\/span> std::<\/span>endl;\r\n\r\n  }\r\n\r\n  std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n  \/\/ std::wstring<\/span>\r\n  std::<\/span>cout <<<\/span> \"std::wstring\"<\/span> <<<\/span> std::<\/span>endl;\r\n  std::<\/span>wsmatch wsmatch;\r\n\r\n  std::<\/span>wstring  wstime{L\"Now it is 00:03.\"<\/span>};\r\n\r\n  if<\/span> (std::<\/span>regex_search(wstime, wsmatch, wrgx)){\r\n\r\n    std::<\/span>wcout <<<\/span> wstime <<<\/span> std::<\/span>endl;\r\n    std::<\/span>wcout <<<\/span> \"Time: \"<\/span> <<<\/span> wsmatch[0<\/span>] <<<\/span> std::<\/span>endl;\r\n\r\n  }\r\n\r\n  std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
First, I used a const char*<\/span>, a std::strin<\/span>g, a const wchar_t*<\/span>, and finally, a std::wstring<\/span> as text. Because it is almost the same code in the four variations, from now on and for the rest of this post, I will only refer to the std::strin<\/span>g.<\/p>\n
The text contains a substring that stands for a time expression. Thanks to the regular expression “([01]?[0-9]|2[0-3]):[0-5][0-9]<\/span>“, I can search for it.  The regular expression defines a time format consisting of an hour and a minute, separated by a colon. Here is the hour and minute part:
<\/span><\/p>\n