This is something I've searched on the forum and couldn't find exact answer.

TLDR: r"^." is matching the very first two character in the input string.

matchtest.d

import std.stdio : writeln;
import std.regex : matchAll;
import std.file  : read;

void main(){

	string input = cast(string)read("example.txt");
	writeln(matchAll(input, r"^."));

}

Input(example.txt)

HelloWorld

Output

rdmd matchtest.d
[["He"]]

https://dlang.org/phobos/std_regex.html#matchAll
https://dlang.org/library/std/file/read.html

On Monday, 11 December 2023 at 05:18:45 UTC, thinkunix wrote:

Yeah, that's true, my mistake, forgot to update the snippet and note properly. Thanks!

import std.stdio : writeln;
import std.regex : matchAll;
import std.file  : read;

void main(){

	string input = cast(string)read("example.txt");
	writeln(matchAll(input, r"^.."));

}

Matches function declarations and captures function names from .d Source Code file

regexcapture.d

import std.stdio : writeln;
import std.regex : matchAll, regex;
import std.file  : read;

void main(){
	string input = cast(string)read("sourcecode.d");

	foreach(match; matchAll(input, regex(r"\b([A-Za-z_]\w*)\s*\([^)]*\)\s*", "g"))){
		writeln(match.captures()[1]);
	}
}

Input(sourcecode.d)

    BOOL WaitNamedPipeA(LPCSTR, DWORD);
    BOOL WaitNamedPipeW(LPCWSTR, DWORD);
    BOOL WinLoadTrustProvider(GUID*);
    BOOL WriteFile(HANDLE, PCVOID, DWORD, PDWORD, LPOVERLAPPED);
    BOOL WriteFileEx(HANDLE, PCVOID, DWORD, LPOVERLAPPED, LPOVERLAPPED_COMPLETION_ROUTINE);
    BOOL WritePrivateProfileSectionA(LPCSTR, LPCSTR, LPCSTR);
    BOOL WritePrivateProfileSectionW(LPCWSTR, LPCWSTR, LPCWSTR);
    BOOL WritePrivateProfileStringA(LPCSTR, LPCSTR, LPCSTR, LPCSTR);
    BOOL WritePrivateProfileStringW(LPCWSTR, LPCWSTR, LPCWSTR, LPCWSTR);

Note: This small input excerpt was taken from a real source code file: https://github.com/dlang/dmd/blob/master/druntime/src/core/sys/windows/winbase.d#L2069-L2078

Output

C:\Users\Windows10\Documents\matchtest>rdmd regexcapture.d
WaitNamedPipeA
WaitNamedPipeW
WinLoadTrustProvider
WriteFile
WriteFileEx
WritePrivateProfileSectionA
WritePrivateProfileSectionW
WritePrivateProfileStringA
WritePrivateProfileStringW

Relevant links:
https://dlang.org/phobos/std_regex.html#regex
https://dlang.org/phobos/std_regex.html#.RegexMatch.captures
https://regexr.com/