python - Error says Unexpected Character, while back referencing in conditional regular expression -

purpose match pattern "abc" or "<abc>" not "<abc" or "abc>". closing angle braces @ end should matched if opening angle braces exist. logic thought work following

(<)?(abc)(?(\1))> 

i'm trying use referencing , conditionals available. error says unexpected character @ second question mark "?" in regular expression. incase needed i'm trying on python.

the syntax conditionals different, try this:

(<)?(abc)(?(1)>|) 

this means:

if (1) (if group 1 matched), match >, otherwise (|), match nothing. if want prevent match of abc>, need negative lookahead:

(<)?(abc)(?(1)>|(?!>)) 

but considered more complex simpler <abc>|(?<!<)abc(?!>) regex posted donal.