Initialise pointer to beginning of string
test look-behind, nothing preceding so fails
advance pointer one place
test look-behind, '{' so fails
advance pointer one place
test look-behind, 'x' so fails
advance pointer one place
test look-behind, '1' so fails
advance pointer one place
test look-behind, '}' so succeeds
test look-ahead, '[' also succeeds
code block encountered, execute
advance pointer one place
test look-behind, '[' so fails
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