Lazy quantifier in .NET regular expression

Question

I tried the following regex replacement:

Regex.Replace("one, two, three, ", ",([.*?]),\s$", ", and$1.");

Which returns

"one, and two, three."

Looking for:

"one, two, and three."

I have a regex that can do this. I don't need help there.

My question: Doesn't the lazily quantified .*? mean it will match as few as possible? If it did (obviously it didn't), it would stop matching at the comma after "two". Does it instead find the first match possible starting from the start of the string?

Update:

first line should read:

Regex.Replace("one, two, three, ", ",(.*?),\s$", ", and$1.");

hwnd · Accepted Answer · 2015-02-10 15:17:40Z

2

To begin with, [.*?] is incorrect. A character class defines a set of characters. Saying, "match one character specified by the class". Therefore, your regex does not do what you expect. You can't wrap a class around .*?; it matches the characters (., *, ?) literally when implemented.

You can change the grouping construct to use a negated character class instead to simply avoid greedily matching the first comma and everything afterwards until the end of the string.

String result = Regex.Replace("one, two, three, ", @"([^,]*),\s$", " and$1.");
Console.WriteLine(result); //=> "one, two, and three."

Note: *? does mean a non-greedy match meaning "zero or more — preferably as few as possible". The way it is used in context here (used with the end of string $ anchor), the token will greedily match the first comma and every single character afterwards; advancing to the next token in the string and continuously keep backtracking until it asserts at the end of the string position.

edited Feb 10, 2015 at 15:17

answered Feb 10, 2015 at 14:28

hwnd

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1 Comment

Joseph Nields Over a year ago

Whoops, I was using [^,] and just replaced the "^," with ".*?" . Sounds like it does just start at the beginning and returns the first match, then. That's what I was wondering. Thanks for the help.

Avinash Raj · Accepted Answer · 2015-02-10 14:21:41Z

1

,.*?,\s$ matches all the characters from the first comma to the last because . matches also the character comma.

,([^,]*),\s$

DEMO

,.*?,\s$ - comma in your regex matches all the commas.
.*? - will do a non-greedy match of all the characters upto
,\s$ - comma and a space followed by end of the line. So we got a match from the first comma to last.

answered Feb 10, 2015 at 14:21

Avinash Raj

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2 Comments

Joseph Nields Over a year ago

"*?" is supposed to be lazy. Doesn't that mean as few as possible? Starting the match at the first comma is not the fewest possible.

Avinash Raj Over a year ago

Note that ^.*?$, ^.*$ both matches the whole line. Here the part folllowing the .*? pattern plays a main role.

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