https://www.rexegg.com/regex-disambiguation.html

✽ Confusing Couples
✽ Lookahead and Lookbehind: (?= … ), (?! … ), (?<= … ), (?<! … )
✽ Non-Capturing Groups: (?: … ) and (?is: … )
✽ Atomic Groups: (?> … )
✽ Named Capture: (?<foo> … ) and (?P<foo> … )
✽ Inline Modifiers: (?isx-m)
✽ Subroutines: (?1)
✽ Recursion: (?R)
✽ Conditionals: (?(A)B) and (?(A)B|C)
✽ Pre-Defined Subroutines: (?(DEFINE)(<foo> … )(<bar> … )) and (?&foo)
✽ Branch Reset: (?| … )
✽ Inline Comments: (?# … )
✽ Code Capsule: (?{…})
✽ PCRE Callouts: (?C…)
✽ Version Check: (?(VERSION>=x) … )
✽ Ignore ALL Unescaped Whitespace: (?xx)
✽ Disable auto-capture: (?n)