Kleene's Star: why does $_ = "a"; s/a*/e/g produce: ee

a* means zero or more instances of: a right?

so why does $_ = "a"; s/a*/e/g produce: ee

Possible answer: it's replacing the string: "a" with: "e" and it's replacing the empty string: "" with: "e" as well. Or it's replacing the mere absence of a letter: a with a letter: e or it's replacing "zero occurrences" of: a with an: e

Ok then, but:

$_ = "b"; s/a*/e/g produces: ebe

It seems to be replacing the empty string to the left of: b and also the empty string to the right of: b

OK. But then why doesn't it do that for: "a" ? Why doesn't it replace the empty string to the left of: a and also the empty string to the right of: a and also the letter: a itself to get: eee ?

There are just as many zero occurrences of: a on the left side as the right side!

regex perl

Edit Report

Using Damian Conway's excellent Regexp::Debugger, I tried this:

perl -MRegexp::Debugger -E '$_ = "a"; s/a*/e/g; say'

And got this output, in case it makes things any clearer, shown in event logging mode. The first pass match running through the replacement yields this set of events:

a               | a*              |   Starting regex match
a               | a*              |     Trying a literal character zero-or-more times (as many as possible)
                | a*              |     Matched
                |                 |   Regex matched in 3 steps

This is showing that the "a" is matched the first time, which gets replaced by "e".

After completing the match the first time, the debugger lets me run a second match from the same program:

                | <~~             |   Back-tracking in regex
                | a*              |   Back-tracked and restarting regex match
                | a*              |     Trying a literal character zero-or-more times (as many as possible)
                | a*              |     Matched
                |                 |   Regex matched in 3 steps

This is showing that the "" after the original "a" (now "e") is matched the second time and replaced with "e".

Unfortunately, either I don't know how to read the output or Regexp::Debugger gets confused at this point or something, but it repeats again, but doesn't do a replacement.

                | <~~             |   Back-tracking in regex
                | a*              |   Back-tracked and restarting regex match
                | a*              |     Trying a literal character zero-or-more times (as many as possible)
                | a*              |     Matched
                |                 |   Regex matched in 3 steps

Anyway, either Perl has m

answered 2012-08-08T00:06:33.157

Kleene's Star: why does $_ = "a"; s/a*/e/g produce: ee

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