Regex Visualizer

/^(?:foo|ba[rz]){1,3}\d+$/gi

sequence n0 [0,24)
  anchor n1 [0,1) ^
  loop n2 [1,20) {1,3}
    group n3 [1,15)
      choice n4 [4,14) 2
        terminal n5 [4,7) foo
        sequence n6 [8,14)
          terminal n7 [8,10) ba
          class n8 [10,14) [rz]
  loop n9 [20,23) +
    class n10 [20,22) \d
  anchor n11 [23,24) $

The everyday shape: a fork with two ways through, wrapped in a loop that may run up to three times, then a second loop over the digit box. Every row carries the half-open offset span it came from, so a box and its characters can be pointed at together. Drawn: 12 boxes, 0 capture groups, no notes.

/(?<year>\d{4})-(?<month>\d{2})(?=\D)/

sequence n0 [0,36)
  named n1 [0,14) 1 year
    loop n2 [8,13) {4}
      class n3 [8,10) \d
  terminal n4 [14,15) -
  named n5 [15,30) 2 month
    loop n6 [24,29) {2}
      class n7 [24,26) \d
  lookaround n8 [30,36) (?=
    class n9 [33,35) \D

Named groups keep both facts a caller needs — the number and the name — on one frame, in source order. The lookahead sits off the main rail because it is a condition checked at that point, not characters the match consumes. Drawn: 10 boxes, 2 capture groups, no notes.

/((((((((((((((((((((((((((((((((\p{Script=Han}+?))))))))))))))))))))))))))))))))/u

sequence n0 [0,80)
  capture n1 [0,80) 1
    capture n2 [1,79) 2
      capture n3 [2,78) 3
        capture n4 [3,77) 4
          capture n5 [4,76) 5
            capture n6 [5,75) 6
              capture n7 [6,74) 7
                capture n8 [7,73) 8
                  capture n9 [8,72) 9
                    capture n10 [9,71) 10
                      capture n11 [10,70) 11
                        capture n12 [11,69) 12
                          capture n13 [12,68) 13
                            capture n14 [13,67) 14
                              capture n15 [14,66) 15
                                capture n16 [15,65) 16
                                  capture n17 [16,64) 17
                                    capture n18 [17,63) 18
                                      capture n19 [18,62) 19
                                        capture n20 [19,61) 20
                                          capture n21 [20,60) 21
                                            capture n22 [21,59) 22
                                              capture n23 [22,58) 23
                                                capture n24 [23,57) 24
                                                  capture n25 [24,56) 25
                                                    capture n26 [25,55) 26
                                                      capture n27 [26,54) 27
                                                        capture n28 [27,53) 28
                                                          capture n29 [28,52) 29
                                                            capture n30 [29,51) 30
                                                              capture n31 [30,50) 31
                                                                capture n32 [31,49) 32
                                                                  opaque n33 [32,48) \p{Script=Han}+?

A deliberate stress vector: 32 capturing groups around a lazy unbounded repeat of a script property. Past 32 levels the layout stops expanding and emits one plain labelled box carrying the source text it stands for, plus a counted note. Drawn: 34 boxes, 32 capture groups, 1 note: N_RAILROAD_OPAQUE.