Ruby Regex * does not match sequence of characters

Question

Why doesn't this regexp match all the bs in the string?

'abbbbbbb'.match(/b*/)
#=> #<MatchData "">

whereas this does:

'abbbbbbb'.match(/b+/)
#=> #<MatchData "bbbbbbb">

The definition of * is: "The asterisk indicates there is zero or more of the preceding element".
The definition of + is: "The plus sign indicates there is one or more of the preceding element".

The only difference between the two operators then is the number of matches that they make. They should both match 7 bs.

Use rubular.com it has a regex application that you can test regex expressions into. Trying putting the b after the * — Schylar, Sep 21 '14 at 06:22
`b*` means 0 or more `b`s. won't there be 2 matches? Nothing and the `bbbb`? — Kamehameha, Sep 21 '14 at 06:28
`'abbbbbbb'[/b*/] => ""` matches an empty string at the beginning of the string. `'bbbbbbb'[/b*/] => "bbbbbbb"`. Note also `'abbbbbbb'.scan(/b*/) => ["", "bbbbbbb", ""]`. — Cary Swoveland, Sep 21 '14 at 06:28
@CarySwoveland, Arup: Whopps, Cary is right. I was wrong. Sorry. — sawa, Sep 21 '14 at 06:29
@sawa Yes mine also was on wrong track. :-) It was a Regexp _googly_ that I missed. ;) — Arup Rakshit, Sep 21 '14 at 06:32

score 4 · Accepted Answer · answered Sep 21 '14 at 06:26

4

It's not a bug and you just answered your question yourself.

The definition of * is: "The asterisk indicates there is zero or more of the preceding element"

So /b*/ matches if there's no b characters at all.

answered Sep 21 '14 at 06:26

user2422869

Ah ha! You're right, thank you. I had been reading it as 'at least 0' and 'at least 1'! – Robin Smith Sep 21 '14 at 06:29
Well, even that would be wrong. In that case i don't know what i was thinking :D – Robin Smith Sep 28 '14 at 02:52

1 Answers1