How do I write this regex to replace a string in perl?

If the string begins with ^abc,don't modify it.otherwise append abc to the beginning of it.

I know i开发者_如何转开发t can be done by 2 steps:m// and s//,but I want to do it in a single s/../ in perl.

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s/^(?!\^abc)/abc/

does the trick, although

$_ = 'abc'.$_ if !/^\^abc/;

might be clearer.

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>perl -E"$_=$ARGV[0]; s/^(?!\^abc)/abc/; say;" "^abcdef"
^abcdef

>perl -E"$_=$ARGV[0]; s/^(?!\^abc)/abc/; say;" "defghi"
abcdefghi

>perl -E"$_=$ARGV[0]; $_ = 'abc'.$_ if !/^\^abc/; say;" "^abcdef"
^abcdef

>perl -E"$_=$ARGV[0]; $_ = 'abc'.$_ if !/^\^abc/; say;" "defghi"
abcdefghi