How do I use std::regex_replace to replace string into lowercase?

Question

I find this regex for replacement Regex replace uppercase with lowercase letters

Find: (\w) Replace With: \L$1

My code

string s = "ABC";
cout << std::regex_replace(s, std::regex("(\\w)"), "\\L$1") << endl;

runs in Visual Studio 2017.

output:

\LA\LB\LC

How do I write the lowercase function mark in C++?

As far as I know, lowercasing is exclusive to Perl-style regex. C++ doesn't have a Perl-style regex syntax option, so I'm guessing it can't. Sure, it's useful for on-the-fly changes in a text editor, but you have a string and you have algorithms that work on strings, so I don't see the point. — Qaz
– Qaz, Commented Nov 2, 2018 at 5:09
@xskxzr, because my purpose is complex. not simplely replace all chars. some like this "I DON'T want to ...", I want to replace "DON'T", but not "I". regex can do this easily. — Zhang
– Zhang, Commented Nov 2, 2018 at 5:10
AFAIK, no such magic like \L$1 in C++. The best shot may be boost regex_replace with a lambda applying tolower() on each character. Or you can use similar technique here: stackoverflow.com/questions/22617209/… — halfelf
– halfelf, Commented Nov 2, 2018 at 5:11

Adrian Mole · Accepted Answer · 2023-12-20 03:39:16Z

1

As halfelf mentioned in their comment, you can use boost. But you do not need to use a callback. The syntax you've given in your question will work when simply replacing std:: with boost::.

I.e., doing:

string s = "ABC";
cout << boost::regex_replace(s, boost::regex("(\\w)"), "\\L$1") << endl;

gives:

abc

edited Dec 20, 2023 at 3:39

Adrian Mole

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answered Dec 18, 2023 at 17:35

RagtimeTime

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Comments

Administrator · Accepted Answer · 2018-11-02 06:33:28Z

Since there is no the magic like \L, we have to take a compromise - use regex_search and manually covert the uppers to lowers.

template<typename ChrT>
void RegexReplaceToLower(std::basic_string<ChrT>& s, const std::basic_regex<ChrT>& reg)
{
    using string = std::basic_string<ChrT>;
    using const_string_it = string::const_iterator;
    std::match_results<const_string_it> m;
    std::basic_stringstream<ChrT> ss;

    for (const_string_it searchBegin=s.begin(); std::regex_search(searchBegin, s.cend(), m, reg);)
    {
        for (int i = 0; i < m.length(); i++)
        {
            s[m.position() + i] += ('a' - 'A');
        }
        searchBegin += m.position() + m.length();
    }
}

void _replaceToLowerTest()
{
    string sOut = "I will NOT leave the U.S.";
    RegexReplaceToLower(sOut, regex("[A-Z]{2,}"));

    cout << sOut << endl;

}

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