Regular expressions are a common tool to perform advanced operations on text, including complex searches, replacements, splitting, and more. Unlike JavaScript or Perl, D does not have regular expressions built into the language, but it has all the same power—and more—as those languages through the std.regex
Phobos module. To explore regular expressions in D, we'll write a small program that searches stdin
for a particular regex that is given on the command line and prints out the matching lines.
Let's use the search operation with regular expressions by executing the following steps:
Regex
object. If you are using string literals, D's r""
or ``
syntax makes it much more readable.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!"); }
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
.
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.
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