Why C++ Regex Not Matching?

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  • Thread starter cheers00
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In summary, the conversation revolves around the use of regular expressions to match a string of characters. The issue arises when the backslash character is not properly escaped, causing the regex engine to interpret it as a special character instead of a literal backslash. The solution is to double up the backslashes in the regex match string.
  • #1
5
0
I am wondering why this is not matching. I tested my regular expression on notepad2 and it matches fine.


std::string line = " 10 0 5 0";
std::regex rgx("\s+\d+\s+\d+\s+\d+\s+\d+");

if (regex_match(line, rgx))
{
// do something...
int a = 2;
}
 
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  • #2
"\s" is a single character string. Containing what character? Who knows. That backslash s is a nonstandard escape sequence. Your code won't even compile on my computer. You need to use "\\s" to denote a backslash followed by an 's'. The same goes for all of your other single backslashes.
 
  • #3
cheers00 said:
I am wondering why this is not matching. I tested my regular expression on notepad2 and it matches fine.


Code:
std::string line = "		10			0		5		0";
std::regex rgx("\s+\d+\s+\d+\s+\d+\s+\d+");

if (regex_match(line, rgx))
 {
        // do something...
	 int a = 2;
 }

The only thing that occurs to me is that the string is terminated with an ASCII null char, possibly looking like \0, and your regex match string doesn't take that into account.

BTW, I replaced your font tags with [code] tags. Now your spaces are appearing.
 
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  • #4
Double up those \. The compiler is interpreting \s as a special character, so the regex engine is seeing something like s+, not \s+. Use \\s and \\d and you'll be fine.

Edit: must type quicker.
 
  • #5
Ibix said:
Double up those \. The compiler is interpreting \s as a special character, so the regex engine is seeing something like s+, not \s+. Use \\s and \\d and you'll be fine.
Your explanation makes more sense than mine...
Ibix said:
Edit: must type quicker.
 
  • #6
Ibix said:
Double up those \. The compiler is interpreting \s as a special character, so the regex engine is seeing something like s+, not \s+. Use \\s and \\d and you'll be fine.

Edit: must type quicker.

doubling back slash worked. thank you
 
  • #7
!{a problem} :D
 

1. Why is my C++ regex not matching anything?

There are a few possible reasons for this. One common reason is that the regular expression pattern is not correctly formatted or does not match the input string. Another possibility is that the flags used in the regex function are not appropriate for the given pattern and input. It is also possible that there is a bug in the code or an error in the logic of the program.

2. How can I debug my C++ regex to see why it is not matching?

One way to debug your C++ regex is to print out the regular expression pattern and the input string to make sure they are what you expect. You can also try using a simpler regular expression to see if it matches, and then gradually add complexity until you find the issue. Additionally, you can use online regex testers to check your regular expression against different inputs and see if it outputs the expected matches.

3. Can the use of special characters in my C++ regex affect the matching?

Yes, special characters in a regular expression can have specific meanings and can affect the matching process. For example, the dot (.) character matches any single character, while the asterisk (*) matches the preceding character zero or more times. It is important to understand the special characters and their meanings in order to use them correctly in your regex patterns.

4. Are there any performance considerations when using C++ regex?

Yes, there are some performance considerations when using C++ regex. Regular expressions can be computationally expensive, especially if they are complex or used on large input strings. It is important to carefully design your regular expressions to be as efficient as possible and to avoid unnecessary backtracking or repetition. You can also consider using other string manipulation methods if performance is a concern.

5. Can I use C++ regex to match multiple patterns in a single string?

Yes, you can use C++ regex to match multiple patterns in a single string. This can be achieved by using alternation (the pipe symbol |) in your regular expression, or by using capturing groups to extract specific parts of the matched string. You can also use a loop to iterate through multiple matches in a given string. It is important to understand the syntax and functionality of regular expressions in order to effectively match multiple patterns in a single string.

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