enter some regular expressions!
α := 
β := 

   ~α = ϕ
   ~β = ϕ
α < β = false
α = β = true
α > β = false
α & β = .*
α ^ β = ϕ
α - β = ϕ

s := hello world

s ∈ α = true
s ∈ β = true

|α| = 0
|β| = 0

dfa(α) has 0 states

dfa(β) has 0 states


regex syntax

  .         match any single character
  xy        concatenation: match x and then y
  x|y       alternation: match x or y
  x*        kleene star: match x zero-or-more times
  (xyz)     grouping: treat xyz as a single item (e.g. (xyz)*)
  ()        an empty regex matches the empty string
  x+        kleene plus: match x one-or-more times (equivalent to xx*)
  x?        optional: optionally match x (equivalent to (x|))
  x{n}      exponentiation: concatenate x to itself n times
  x{m,n}    repetition: concatenate x to itself between m and n times
  [a-z0-9]  grouping: match any single character in the group
  [^a-z0-9] negative grouping: match any single character not in the group
  \c        escaping: match the special character c
  \u001a    unicode escaping: match the corresponding UTF-16 character
  a, b, c   all other characters match themselves

unsupported features

  - anchors (e.g. ^, $), although ^ and $ must still be escaped!
  - zero-width assertions (e.g. (?=...), (?<=...))
  - back references (e.g. \1, \2)
  - subgroup extraction
  - searching or partial matching
  - other flags that change behavior (e.g. case-insensitivity)

see https://github.com/non/antimirov for more information

by eiríkr åsheim (@d6 on twitter and mastodon)