Perl i Modifier
The ignore-case (or case-insensitive) modifier (i) is used to match patterns without regard to case.
Thus:
/pattern/i
will match any of these successfully:
Pattern
PATTERN
PaTtErN
modifier: A character placed after a match or substitution pattern that modifies the matching process.
For example, the i modifier makes the match case-insensitive. Also called a flag.