FWIW, v5.12.2 has no trouble "parsing" this regex 
Compiling REx "%n    ( # start of bracket 1%n    {{ # match an opening
+ {{ b"...
Final program:
   1: OPEN1 (3)
   3:   EXACT <{{> (5)
   5:   CURLYX[0] {0,32767} (30)
   7:     BRANCH (24)
   8:       SUSPEND (29)
  10:         PLUS (22)
  11:           ANYOF[\0-z|~-\377][{unicode_all}] (0)
  22:         SUCCEED (0)
  23:       TAIL (28)
  24:     BRANCH (FAIL)
  25:       GOSUB1[-24] (29)
  28:     TAIL (29)
  29:   WHILEM[2/1] (0)
  30:   NOTHING (31)
  31:   EXACT <}}> (33)
  33: CLOSE1 (35)
  35: END (0)
anchored "{{" at 0 floating "}}" at 2..2147483647 (checking floating) 
+minlen 4

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and (?1) # recurse to bracket 1 appears to be a feature introduced in 5.10 (?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)

In reply to Re^2: ERROR ... nested quantifiers in regex by Anonymous Monk
in thread ERROR ... nested quantifiers in regex by &#350;uRvīv&#337;r

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