Question

How can I transform the string “a b a c d a a” into the string “1 b 2 c d 3 4” with a regular expression?

Is this possible? Preferred flavor is perl, but any other will do too.

s/a/ \????? /g

Answer 1

This substitution will do it.

$ perl -p -e 's/a/++$i/ge' 
a b a c d a a
1 b 2 c d 3 4

• e Evaluate the right side as an expression.
• g Replace globally, i.e., all occurrences.