There is more to write about the usage of regular expressions than I wrote in my last post The Regular Expression Library. Let’s continue.
The text determines the regular expression, the result, and the capture groups
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.
The table gives all the types depending on the text type.
Here is an example of all the variations of std::regex_search depending on the text type.
First, I used a const char*, a std::string, a const wchar_t*, and finally, a std::wstring 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::string.
The text contains a substring that stands for a time expression. Thanks to the regular expression “(?[0-9]|2[0-3]):[0-5][0-9]“, 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:
- hour: ?[0-9]|2[0-3]:
- ?: 0 or 1 (optional)
- [0-9]: a number from 0 to 9
- |: stands for or
- 2[0-3]: 2 followed by a number from 0 to 3
- minute: [0-5][0-9]: a number from 0 to 5 followed by a number from 0 to 9
Finally, the output of the program.
Use regex_iterator or regex_token_iterator for repeated search
Don’t repeat std::search calls because you can quickly lose word boundaries or have empty hits. Use std::regex_iterator or std::regex_token_iterator for repeated search instead. std::regex_token_iterator allows you to address each capture group’s components or the text between the matches.
The “Hello World” of repeated search with regex is to count how often a word appears in a text. Here is the corresponding program.
To better understand the program, I added a few comments to it.
The using declarations in line 1 help me to type less. The function wordCount determines the frequency of each word, and the function frequencyOfWords return to each frequency of all words. What is a word? Line 2 defines it with the regular expression, and line 3 uses it in a std::sregex_iterator. Let’s see which answers I can give with the two functions.
- Line 4: first 20 (key, value)-pairs
- Line 5: frequency of the words “Web” and “The”
- Line 6: number of unique words
- Line 7: number of frequencies
- Line 8: all appearing frequencies
- Line 9: the most frequently used word
- Line 10: words that appear more than 1000 times
Now, I need a lengthy text. Of course, I will use Grimm’s fairy tales from the project Gutenberg . Here is the output:
I’m almost done with the regex functionality in C++, but I have one guideline in mind which makes repeated search often easier: Search not for the text patterns but the delimiters of the text patterns. I call this a negative search.
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