add explanatory comment

packages/block-serialization-default-parser/src/index.js

@@ -2,6 +2,45 @@ let document;
 let offset;
 let output;
 let stack;
+
+/**
+ * Matches block comment delimiters
+ *
+ * While most of this pattern is straightforward the attribute parsing
+ * incorporates a tricks to make sure we don't choke on specific input
+ *
+ *  - since JavaScript has no possessive quantifier or atomic grouping
+ *    we are emulating it with a trick
+ *
+ *    we want a possessive quantifier or atomic group to prevent backtracking
+ *    on the `}`s should we fail to match the remainder of the pattern
+ *
+ *    we can emulate this with a positive lookahead and back reference
+ *    (a++)*c === ((?=(a+))\1)*c
+ *
+ *    let's examine an example:
+ *      - /(a+)*c/.test('aaaaaaaaaaaaad') fails after over 49,000 steps
+ *      - /(a++)*c/.test('aaaaaaaaaaaaad') fails after 85 steps
+ *      - /(?>a+)*c/.test('aaaaaaaaaaaaad') fails after 126 steps
+ *
+ *    this is because the possessive `++` and the atomic group `(?>)`
+ *    tell the engine that all those `a`s belong together as a single group
+ *    and so it won't split it up when stepping backwards to try and match
+ *
+ *    if we use /((?=(a+))\1)*c/ then we get the same behavior as the atomic group
+ *    or possessive and prevent the backtracking because the `a+` is matched but
+ *    not captured. thus, we find the long string of `a`s and remember it, then
+ *    reference it as a whole unit inside our pattern
+ *
+ *    @cite http://instanceof.me/post/52245507631/regex-emulate-atomic-grouping-with-lookahead
+ *    @cite http://blog.stevenlevithan.com/archives/mimic-atomic-groups
+ *    @cite https://javascript.info/regexp-infinite-backtracking-problem
+ *
+ *    once browsers reliably support atomic grouping or possessive
+ *    quantifiers natively we should remove this trick and simplify
+ *
+ * @type RegExp
+ */
 const tokenizer = /<!--\s+(\/)?wp:([a-z][a-z0-9_-]*\/)?([a-z][a-z0-9_-]*)\s+({(?:(?=([^}]+|}+(?=})|(?!}\s+\/?-->)[^])*)\5|[^]*?)}\s+)?(\/)?-->/g;
 
 function Block( blockName, attrs, innerBlocks, innerHTML, innerContent ) {