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How to remove everything after specific character in string using Java

I have a string that looks like this:
analitics#gmail.com#5
And it represents my userId.
I have to send that userId as parameter to the function and send it in the way that I remove number 5 after second # and append new number.
I started with something like this:
userService.getUser(user.userId.substring(0, userAfterMigration.userId.indexOf("#") + 1) + 3
What is the best way of removing everything that comes after the second # character in string above using Java?

Here is a splitting option:
String input = "analitics#gmail.com#5";
String output = String.join("#", input.split("#")[0], input.split("#")[1]) + "#";
System.out.println(output); // analitics#gmail.com#
Assuming your input would only have two at symbols, you could use a regex replacement here:
String input = "analitics#gmail.com#5";
String output = input.replaceAll("#[^#]*$", "#");
System.out.println(output); // analitics#gmail.com#

You can capture in group 1 what you want to keep, and match what comes after it to be removed.
In the replacement use capture group 1 denoted by $1
^((?:[^#\s]+#){2}).+
^ Start of string
( Capture group 1
(?:[^#\s]+#){2} Repeat 2 times matching 1+ chars other than #, and then match the #
) Close group 1
.+ Match 1 or more characters that you want to remove
Regex demo | Java demo
String s = "analitics#gmail.com#5";
System.out.println(s.replaceAll("^((?:[^#\\s]+#){2}).+", "$1"));
Output
analitics#gmail.com#
If the string can also start with ##1 and you want to keep ## then you might also use:
^((?:[^#]*#){2}).+
Regex demo

The simplest way that would seem to work for you:
str = str.replaceAll("#[^.]*$", "");
See live demo.
This matches (and replaces with blank to delete) # and any non-dot chars to the end.

How to trim/cut string in java by symbol?

I'm working on a project where my API returns url with id at the end of it and I want to extract it to be used in another function. Here's example url:
String advertiserUrl = http://../../.../uuid/advertisers/4 <<< this is the ID i want to extract.
At the moment I'm using java's string function called substring() but this not the best approach as IDs could become 3 digit numbers and I would only get part of it. Heres my current approach:
String id = advertiserUrl.substring(advertiserUrl.length()-1,advertiserUrl.length());
System.out.println(id) //4
It works in this case but if id would be e.g "123" I would only get it as "3" after using substring, so my question is: is there a way to cut/trim string using dashes "/"? lets say theres 5 / in my current url so the string would get cut off after it detects fifth dash? Also any other sensible approach would be helpful too. Thanks.
P.s uuid in url may vary in length too

You don't need to use regular expressions for this.
Use String#lastIndexOf along with substring instead:
String advertiserUrl = "http://../../.../uuid/advertisers/4";// <<< this is the ID i want to extract.
// this implies your URLs always end with "/[some value of undefined length]".
// Other formats might throw exception or yield unexpected results
System.out.println(advertiserUrl.substring(advertiserUrl.lastIndexOf("/") + 1));
Output
4
Update
To find the uuid value, you can use regular expressions:
String advertiserUrl = "http://111.111.11.111:1111/api/ppppp/2f5d1a31-878a-438b-a03b-e9f51076074a/adver‌​tisers/9";
// | preceded by "/"
// | | any non-"/" character, reluctantly quantified
// | | | followed by "/advertisers"
Pattern p = Pattern.compile("(?<=/)[^/]+?(?=/adver‌​tisers)");
Matcher m = p.matcher(advertiserUrl);
if (m.find()) {
System.out.println(m.group());
}
Output
2f5d1a31-878a-438b-a03b-e9f51076074a

You can either split the string on slashes and take the last position of the array returned, or use the lastIndexOf("/") to get the index of the last slash and then substring the rest of the string.

Use the lastIndexOf() method, which returns the index of the last occurrence of the specified character.
String id = advertiserUrl.substring(advertiserUrl.lastIndexOf('/') + 1, advertiserUrl.length());

Replace querystring attribute value in java using regex

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.

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.

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.

Regex for parsing string with same type of expression

I am new to regex parsing in java. I want to parse the string which contain the records. But I want to select the selected part of that record only.
\"6\":\"Services Ops\",\"practice_name\":\"Services Ops\",\"7\":\"Management\",
For this, I have written regex expression as
(^\\\"6\\\":\\\"[A-Za-z \s]*)
and above expression gives me result as : \"6\":\"Services Ops\
I want only Service Ops
And also there are multiple records like \"5"\:\"xxx"\ and so on thus if I write the expression for only Service Ops then entries from other fields are also included in the result of the expression.
Is there any way that we can select the string which start with some pattern but we can exclude that pattern.
Like in above example, string starting with \"6\":\" but we can exclude this part and get only Service Ops as result.
Thank you.

You can use lookarounds which perform only a check but don't match:
lookahead (?=...)
lookbehind(?<=...)
example:
(?<=\\\"6\\\":\\\")[^\"]++(?=\")
An another way is to use a capturing group (...):
\\\"6\\\":\\\"([^\"]++)\"
Then you can extract only the content of the group. Example:
Pattern p = Pattern.compile("\\\"6\\\":\\\"([^\"]++)\"");
Matcher m = p.matcher(yourString);
if (m.matches()) {
System.out.println(m.group(1));
}

Java String Replace Regex

I am doing some string replace in SQL on the fly.
MySQLString = " a.account=b.account ";
MySQLString = " a.accountnum=b.accountnum ";
Now if I do this
MySQLString.replaceAll("account", "account_enc");
the result will be
a.account_enc=b.account_enc
(This is good)
But look at 2nd result
a.account_enc_num=a.account_enc_num
(This is not good it should be a.accountnum_enc=b.accountnum_enc)
Please advise how can I achieve what I want with Java String Replace.
Many Thanks.

From your comment:
Is there anyway to tell in Regex only replace a.account=b.account or a.accountnum=b.accountnum. I do not want accountname to be replace with _enc
If I understand correctly you want to add _enc part only to account or accountnum. To do this you can use
MySQLString = MySQLString.replaceAll("\\baccount(num)?\\b", "$0_enc");
(num)? mean that num is optional so regex will accept account or accountnum
\\b at start mean that there can be no letters, numbers or "_" before account so it wont accept (affect) something like myaccount, or my_account.
\\b at the end will prevent other letters, numbers or "_" after account or accountnum.

It's hard to extrapolate from so few examples, but maybe what you want is:
MySQLString = MySQLString.replaceAll("account\\w*", "$0_enc");
which will append _enc to any sequence of letters, digits, and underscores that starts with account.

try
String s = " a.accountnum=b.accountnum ".replaceAll("(account[^ =]*)", "$1_enc");
it means replace any sequence characters which are not ' ' or '=' which starts the word "account" with the sequence found + "_enc".
$1 is a reference to group 1 in regex; group 1 is the expression in parenthesis (account[^ =]+), i.e. our sequence
See http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html for details