Replace querystring attribute value in java using regex

Question

I want to replace querystring value but it's creating some problems:

Problem 1: Its Removing the "&" symbol after replacing

String queryString = "?pid=1&name=Dell&cid=25";
String nQueryString=queryString.replaceAll("(?<=[?&;])pid=.*?($|[&;])","pid=23");
System.out.println(nQueryString);

output of above example ?pid=23name=Dell&cid=25

you can see its removed the "&" after pid

Problem 2: Its not working if I removed the "?" symbol from the queryString variable.

String queryString = "pid=1&name=Dell&cid=25";
String nQueryString=queryString.replaceAll("(?<=[?&;])pid=.*?($|[&;])","pid=23");
System.out.println(nQueryString);

output of above example ?pid=1&name=Dell&cid=25

We can say the regex is not working, So anyone can suggest me better regex which completely fulfill my requirements.

Steve P. · Accepted Answer · 2013-12-09 14:10:49Z

1

queryString.replaceAll("(?<=[?&;])pid=.*?(?=[&;])", "pid=23")

Difference is that I'm using a positive-lookahead: (?=[&;]), which is zero-length, making it atomic, and is not actually included in the replacement via replaceAll(), just like your original positive-lookbehind is not replaced.

Alternatively, we can match until a & or ; is found, but not included in the replacement, ie:

queryString.replaceAll("(?<=[?&;])pid=[^&;]*", "pid=23")

[^&;] : ^ negates the following: &;, so [^&;]* will match until a ; or & is encountered.

Yours does not work because ($|[&;]) is a non-atomic group, specifically a capturing group, and thus is included in the replacement. NB: a non-capturing group (?:$|[&;]) would also fail here.

To your final note, you're using a positive look-behind for ?, &, and ;, so by removing the ?, it will no longer match, which makes sense.

edited Dec 9, 2013 at 14:10

answered Dec 9, 2013 at 13:59

Steve P.

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2 Comments

Anil Kumar Pandey Over a year ago

Yes @Steve you are right, but when I was trying to replace the same queryString with blank space then its not removing "&" symbol. queryString.replaceAll("(?<=[?&;])pid=[^&]*","") will give ?&name=Dell&cid=25

Anil Kumar Pandey Over a year ago

Oh yes, I just wanted to replace the queryString values, but I just tried to replace the attribute. I would be better if the same regex will work for me, that's why I tried and commented. Thanks @Steve, your answer is very useful for me.

anubhava · Accepted Answer · 2013-12-09 13:56:09Z

0

Use this regex instead:

String nQueryString = queryString.replaceAll("(?<=[?&;])pid=[^&]*", "pid=23");
//=> ?pid=23&name=Dell&cid=25

Here [^&]* is called negation matching pattern, that will match query string value until & is found OR else end of string is found thus leaving rest of the query string un-effected.

answered Dec 9, 2013 at 13:56

anubhava

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