Regular Expressions

Regular expressions are enclosed in matching pound sign (#) characters with a prepended re: re#...#. KRL regular expressions follow the JavaScript standard, which closely follows the conventions for Perl regular expressions. The following modifiers may appear in any order after the closing character:

  • i. The i modifier makes the regular expression case insensitive.
  • g. The g modifier applies the regular expression globally.

For example, the following code replaces the first instance of foo in p with bar:

p.replace(re#foo#, "bar")

In contrast, the following code replaces all instances of foo in p with bar:

p.replace(re#foo#g, "bar")

Special characters

Like strings, the only special characters are the terminator (#) and the backslash (/). To use pound signs or backslashes inside regular expressions, escape them with backslashes:

re#\#\\# // '#' followed by '\'

A newline (\n) requires a line break:


Other characters can be inserted literally (some text editors are better at this than others), or consider using chr() and converting the string to a regular expression.


KRL uses the hash character to delimit regular expressions instead of the more common (and acceptable) slash (/) because the slash is a frequently used character in URL paths. This removes the need to quote the slash with backslashes: re/\/archives\/\d{4}\//. Using an alternate delimiter makes the regex more readable and thus communicates its meaning more clearly: re#/archives/\d{4}/#.


Some regular expressions found in KRL code "in the wild".


select when d1 t1 time re#^(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}[.]\d{3}Z)$# setting(timestamp)

The regular expression above matches a string such as produced by the time:now() function in the time library. Notice that it expects the string to consist entirely, from beginning (^) to end ($) of an ISO8601 time string and furthermore, that string is captured (note the surrounding pair of parentheses). In this context, a rule controlled by the event expression shown above would be selected when an incoming event had domain d1, type t1, and an event attribute named "time" which satisfied the ISO8601 pattern specified by the regular expression. Within the rest of the rule the validated value will be available as the value bound to the name timestamp.



The regular expression above matches an event channel identifier, which in the Node.js pico engine is a distributed identifier or DID.

non-negative integer


non-empty string of characters


a literal decimal point


This appeared in a function to compute the integer part of a non-negative value which might contain a decimal point:

      math_int = function(num) {
        val ="String");
        dec = val.match(re#[.]#);
        dec => val.extract(re#(\d*)[.]\d*#)[0].as("Number") | num;
      tens = math_int( x / 10 )

The function uses the string operator extract to isolate the numerator from the number computed as x / 10.