(?s)applies the following effective flags to the remainder of the pattern: s s modifier: single line. Allows a dot to match line terminators
Positive Lookbehind(?<=^(?!(?:.*LT circa){2}).*LT circa\s*) ^start of string
Negative Lookahead(?!(?:.*LT circa){2}) Non-Capturing Group(?:.*LT circa){2} {2}repeats the group contents below exactly 2 times:
.*any Unicode character, repeated any number of times
LT circathe literal text LT circa (case sensitive) .*any Unicode character, repeated any number of times
LT circathe literal text LT circa (case sensitive) \s*a Unicode whitespace character, repeated any number of times
\d{1,2}a Unicode decimal digit, repeated between 1 and 2 times
\.the literal . (4610 / 568 / 2E16)
\d{1,2}a Unicode decimal digit, repeated between 1 and 2 times
\.the literal . (4610 / 568 / 2E16)
\d{4}a Unicode decimal digit, repeated exactly 4 times