fix variable lookup parse timing out with missing closing bracket

[ggmichaelgo]

Problem

This PR #1680 has introduced a bug that causes VariableLookup#initialize to time out.
This happened when the VariableLookup's markup had a missing closing bracket and a very long string.

# this will execute within 50ms
"[12345678901234567890".scan /\[(?:[^\]\[]+|\g<0>)*\]/

# this will take a second to execute
"[123456789012345678901234".scan /\[(?:[^\]\[]+|\g<0>)*\]/

The regex is taking a long time to execute because the non-capturing group (?:[^\[^]+... recursively re-searches/backtraces the input with different lengths.
The non-capturing group match will look like this throughout the search:

1. [123456789012345678901234
2. [12345678901234567890123
3. [1234567890123456789012
4. [123456789012345678901
5. [12345678901234567890
6. and so on...

Solution

We can use the atomic group feature in regex to prevent regex from dividing the non-bracket characters, and early end the search.

\[(?:[^\[\]]+|\g<0>)*\]

\[          # match open bracket
(?>         # start an atomic capturing group (prevent backtracking)
  [^\[\]]+  # match non-square brackets
  |         # OR
  \g<0>     # go back to the beginning of the regex
)*          # end of non-capturing group
\]          # match closing bracket