Farkle's string regexes

In Farkle, terminals are defined by regular expressions or regexes. Defining a non-trivial regex used to take several lines of code like this example of a number with an optional sign at the beginning:

open Farkle.Builder

let number = concat [
    chars "+-" |> optional
    plus Number

Not anymore. Starting with Farkle 6, a regex can be defined much more simply and intuitively with a string. Here is the previous example, using a string regex:

open Farkle.Builder

let number = regexString "[+-]?\d+"

And in C#:

using Farkle.Builder;

var number = Regex.FromRegexString("[+-]?\\d+");

These regexes are full-blown Regex-typed objects. They are composable, reusable and can be used anywhere instead of constructed regexes. Despite their similarity however, the language of regex strings is not the same with the language of popular regex libraries like PCRE or .NET's own System.Text.RegularExpressions.Regex. In this guide we will take a look at what is supported in regex strings, what isn't and what is different. So, are you ready? Let's do this!

Supported string regex constructs

Character classes

In Farkle's string regexes, you can define character classes mostly in the same way with PCRE regexes Here's what is supported:

Note: Prior to Farkle 6.2.0, single quotes could be escaped with \'. After that version the regex parser was improved but some construct like that are no longer possible to maintain ambiguity. \ is not anymore specially treated in literal strings.


As with PCRE regexes, quantifiers like the *, + or ? mean "zero or more", "one or more", and "zero or one" respectively. Less known quantifiers like {m,n}, {m,} and {m} mean "between m and n times", "at least m times" and "exactly m times" respectively.

You can also stack quantifiers; \d{4}? will match either four decimal digits or none.

Note: Prior to Farkle 6.2.0, the regex above did not work due to a bug; you had to write (\d{4})?.

Precedence and grouping

The regex disjunction operator | takes precedence over regex concatenation, which means that foo|bar matches either foo or bar, not fo, either o or b, and then ar. You can specify a custom operator precedence with parentheses. For example, fo(o|u) matches only either foo or fou.

Note: Parentheses exist only for defining operator precedence. Capturing groups is not supported.


The dot regex

When I was describing the . regex, I intentionally told it matches any other character and not any character. In other words, the . regex is matched only if no other regex can be matched. The difference is subtle but can have a difference in certain scenarios.

Let's take a look at a simple regex for a string enclosed in double quotes that also supports escaping them: "(.|\")*".

Note: You will need additional escaping to write the above regex in code.

The dot in the above regex will be never matched to a double quote because it also can be matched to the double quote at the end which has a higher priority. In essence, the regex above is the equivalent to "([^"]|\")*".

Now, what if we required the string to have at least one character? The regex would have turned into "(.|\")+".

But the regex above would match strings like ""foo". The reason to this is actually surprisingly simple. Generally x+ is equivalent to xx*, making the regex above equivalent to "(.|\")(.|\")*". In ""foo", the first double quote is matched to the first double quote in the regex, the second one is matched to the regex's first dot, and the third is matched to the regex's final double quote. So if you want a regex that matches strings with at least one character you have to explicitly write "([^"]|\")+".


In Farkle's string regexes, you can have arbitrary whitespace everywhere except of literal strings and character sets and ranges. This means that f o o ( bar ) ? is equivalent to foo(bar)?. If you want to match a literal space you can escape the space (\) or use a character set ([ ]).

This deliberate deviation from the typical regex syntax was made due to Farkle's philosophy that whitespace is automatically handled by default, and allows you to write big regexes in a more clean and less compact way.


When using \ in regexes, be careful with the string escaping performed by programming languages themselves. To match a decimal digit, F# allows you to write an unrecognized escape sequence like "\d", but C# doesn't, failing with an error and you have to use a verbatim string like @"\d".

In a more complicated example, if you want to match the literal sequence of characters \d, the regex is either '\d' or \\d, which you would write as either "'\\d'" or "\\\\d", or as either @"'\d'" or @"\\d" with a verbatim string.

Similarly, writing "\n" somewhere in a regex will be ignored because it is whitespace, as we saw earlier. If you want to match the literal sequence of characters \n, you would follow the example we saw in the previous paragraph. If you want to match an actual line feed character, you would either write it with a literal string as "'\n'", or with a character set as "[\n]".

Unicode categories

Matching characters that belong in a Unicode category is not yet possible. Support might be added in a future version of Farkle if there is demand for it.

How do they work

Finally, let's take a look at how string regexes work. It's actually surprisingly simple. These strings are parsed and converted to constructed regexes using Farkle itself. That parsing happens when you build a designtime Farkle containing a string regex. If a syntax error occurs in a regex string, building the designtime Farkle will fail.

You can parse strings into regular expressions yourself by using the objects in the Farkle.Builder.RegexGrammar module.

So I hope you enjoyed this little tutorial. If you did, don't forget to give Farkle a try and maybe you feel especially quantified today and want to hit the star button as well. I hope that all of you have a wonderful day, and to see you soon. Goodbye!

val number : obj