代码
preg_match('/{{Infobox Item.+?^}}$/sm', $subject, $regs)
正则表达式
{{Infobox Item.+?^}}$

https://regex101.com/r/gC0oV1/1
人类可读
# {{Infobox Item.+?^}}$
#
# Options: Case sensitive; Exact spacing; Dot matches line breaks; ^$ match at line breaks; Greedy quantifiers
#
# Match the character string “{{Infobox Item” literally (case sensitive) «{{Infobox Item»
# Match any single character «.+?»
# Between one and unlimited times, as few times as possible, expanding as needed (lazy) «+?»
# Assert position at the beginning of a line (at beginning of the string or after a line break character) (line feed) «^»
# Match the character string “}}” literally «}}»
# Assert position at the end of a line (at the end of the string or before a line break character) (line feed) «$»