Learning PEGs in Janet

Posted on 2025-10-21.

PEGs in Janet slides

• Number literal

(peg/match '(capture 0) "abc") #-> ""
(peg/match '(capture 1) "abc") #-> "a"
(peg/match '(capture 2) "abc") #-> "ab"
(peg/match '(capture 3) "abc") #-> "abc"
(peg/match '(capture 4) "abc") #-> nil
(peg/match '(capture -1) "abc") #-> nil

• 'foo matches the string "foo" itself
• (sequence "a" "b" "c") or (* "a" "b" "c") -> "abc"
• (choice "a" "b" "c") or (+ "a" "b" "c") -> "a" or "b" or "c"

~(repeat 3 :w) "abcd" #-> "abc"
~(repeat 3 :w) "ab" #-> nil

~(between 2 3 :w) "abcd" #-> "abc"
~(between 2 3 :w) "ab" #-> "ab"
~(between 2 3 :w) "a" #-> nil

• Split by chars

~(any (capture :w)) "abc" #->