in reply to You won't believe what this regular expression does!

Surprising, yes, but I guess I'd call it a pitfall of both * and a zero-width assertion. s/o*$/O/gm first matches "hello\n" and the $ assertion is satisfied, and since it's zero-width the matching position is left just before the \n. Then it matches "hello \n" because o* and $ are satisfied, and then the regex engine matches the same thing again but the rules in Repeated Patterns Matching a Zero length Substring kick in, advancing the position past the \n instead. Update: You can see this in action with: perl -MRegexp::Debugger -e '"hello\nfoo"=~s/o*$/O/gmr' Update 2: Note the same thing happens again with foo - it first replaces oo with O, and then adds the second O because it matched the $ again. You can see this with "o\no".