sed on OS X - can't seem to use + in regexps

Question

Now according to all the literature

echo 1234abcd|sed "s|[0-9]\+|#|g" 

should output #abcd. And

echo abcd|sed "s|[0-9]\+|#|g" 

should output abcd.

But on OS X 10.4.11 the first expression outputs 1234abcd. Using * instead of + works for the first example but fails on the second, outputting #abcd, because the [0-9] pattern is matched zero times.

Does the + operator not work in regular expressions in OS X? Is there an alternative?

Thanks

Answer 1

On OSX, sed by default uses basic REs. You should use sed -E if you want to use modern REs, including the "+" one-or-more operator.

See here for the indication that sed uses basic REs by default, here for the modern RE syntax, and here for the basic RE (ed) information.

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Alternatively, if you have a regular expression engine that doesn't support + at all, you can simply use * instead, by converting (for example):

[a-z]+ 

into:

[a-z][a-z]* 

Answer 2

Obsolete basic regular expressions do not support + and ? quantifiers. They are regular characters.

Alternatives for [0-9]+ are e.g. [0-9]{1,} or [0-9][0-9]*.

Or you can use sed -E to use modern, extended regular expressions.