regex lookbehind followed by lookahead

Question

INPUT: garbage="stff",start1="allshortsofCharactersExceptDoubleQuotes",start2="*&^%$blahblah"

DESIRED RESULT: allshortsofCharactersExceptDoubleQuotes

*&^%$blahblah

Using c# .NET:

string myRegExString = @"(?<=start[0-9].).*(?="")"

Yeilds: allshortsofCharactersExceptDoubleQuotes",start2="*&^%$blahblah

Through testing I know that if I replaced .* with a set that had all the characters except double quotes I would get the desired result but that is a lot of work and I will get that wrong. Also using (?!"") or (?!="") before .* does not work either.

So how do I get the lookahead to stop on the first double quote it finds?

Correct Answers (as far as I tested) from the responses:

(?<=start\d+="")[^""]*(?="")

OR

(?<=start\d+="")[^""]+(?="")

OR this works too but is not quite what was asked for.

(?<=start\d+="")[^""]*

Thanks. I was so wrapped up in the lookahead aspect of this item.

score 1 · Answer 1 · answered Jul 12 '13 at 05:34

1

You should use lazy quantifier .*? which would match as less as possible..In your case .* would match as much as possible and hence it would capture till last "

(?<=start\d+="").*?(?="")

You could get a list of such of values using this code

List<string> output=Regex.Matches(input,regex)
                         .Cast<Match>()
                         .Select(x=>x.Value)
                         .ToList();

answered Jul 12 '13 at 05:34

Anirudha

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thanks. will try the Linq another time. did not test this out – frakenweenie Jul 12 '13 at 13:37

score 0 · Answer 2 · answered Jul 12 '13 at 05:35

0

The problem with your regular expression is that .* is matching too much text. You can make your regular expression lazy by adding a question mark after the star like '.*?' Or you can change it to match every thing except double quoutes with: '[^"]*' which is what I would choose in this case. The following should work. Not tested

string myRegExString = @"(?<=start[0-9].)[^""]*(?="")"

The other solution I suggests is:

string myRegExString = @"(?<=start[0-9].).*?(?="")"

answered Jul 12 '13 at 05:35

rbernabe

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the first is not going to match anything since you are missing `"`..in second your result would include 1st `"` – Anirudha Jul 12 '13 at 06:09
Please see http://stackoverflow.com/questions/1928909/in-c-can-i-escape-a-double-quote-in-a-verbatim-string-literal The double quote is escaped by an extra double quote in string lterals – rbernabe Jul 12 '13 at 06:17
Thanks! Both of those worked. I got so wrapped up in the lookahead I could not see the obvious :) – frakenweenie Jul 12 '13 at 13:40

score 0 · Answer 3 · answered Jul 12 '13 at 05:40

0

You can use this:

@"(?<=start\d="")[^""]+(?="")"

the result is the whole pattern.

answered Jul 12 '13 at 05:40

Casimir et Hippolyte

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@Anirudh: yes and no, because this checks there's a double quote after. Imagine with this string:`abcd start2="abcd`, what should i do? – Casimir et Hippolyte Jul 12 '13 at 06:11
thanks. Yes the (?="") is redundant but is what I asked for and demonstrates that a lookahead can be used if required. – frakenweenie Jul 12 '13 at 13:41

regex lookbehind followed by lookahead

3 Answers3