How to do it…

Let's use the search operation with regular expressions by executing the following steps:

Pretty simple! The code is as follows:

void main(string[] args) {
    import std.regex, std.stdio;
    auto re = regex(args[1], "g");
    foreach(line; stdin.byLine)
    if(line.match(re)) writeln(line, " was a match!");
}

How it works…

Regular expressions in D are a library type instead of a built-in type, but this doesn't affect their usability, their speed, or their simplicity. Declare a regular expression with the regex(pattern, flags) helper function. If you are using a string literal for the pattern, it will look nicer if you use one of D's raw string syntaxes, either r"pattern here" or `pattern here`. In those strings, the backslash doesn't have to be escaped, allowing the regular expression to look natural. The pattern syntax in D is almost similar to the syntax used in JavaScript.

After the regex is declared, you can use methods from std.regex to do all the basic tasks. The string.matchAll(pattern) function returns a list of matches in the string. You can check it in an if statement to see if it matches at all, or you can perform a foreach loop over it to get the details of the match.

The std.regex function also provides methods to replace instances, including a callback function, or to split a string. These functions work in the same way as replace and split from std.string.

There's more…

D's regular expressions are able to outperform most other regex engines by being processed at compile time. Instead of writing regex, write ctRegex! and make sure that the variable is static. The regular expression and the usage code do not have to be changed, as shown in the following code:

static ex = ctRegex!`[0-9]{3}-[0-9]{4}`;
foreach(match; "My phone number is 123-4567!".match(ex)) writeln(match);

The output is as follows:

["123-4567"]

With ctRegex, std.regex will then create D code to implement your specific regular expression and compile it, generating a fully optimized function to implement it. We'll learn how to create this kind of code later in the book.

See also