Using Regular Expressions to Extract specific Values in Java

Question

I have several strings in the rough form:

String s = "Rendering content from websiteNAme using user agent userAgentNameWithSpaces ; for user username ; at time someTime";

I want to extract the values for websiteName, userAgentNameWithSpaces, username and someTime. I have tried the following code.

private static final Pattern USER_NAME_PATTERN = Pattern.compile("for user.*;");
final Matcher matcher = USER_NAME_PATTERN.matcher(line); 
matcher.find() ? Optional.of(matcher.group(group)) : Optional.empty();

It returns the whole string " for user username" after that I have to replace the for user string with empty string to get the user name. However, I want to know if there is regex to just get the username directly?

Have you tried anything? Also is your String format the same every time? — brso05
– brso05, Commented Apr 19, 2017 at 19:15

score 2 · Accepted Answer · 2017-04-20 12:08:02Z

2

You can use regex groups:

Pattern pattern = Pattern.compile("for user (\\w+)");
Matcher matcher = pattern.matcher(s);
if (matcher.find()) {
    System.out.println(matcher.group(1));
}

The pair of parenthesis ( and ) forms a group that can be obtained by the matcher using group method (as it's the first parenthesis, it's group 1).

\w means a "word character" (letters, numbers and _) and + means "one or more ocurrences". So \w+ means basically "a word" (assuming your username has only these characters). PS: note that I had to escape \, so the resulting expression is \\w+.

The ouput of this code is:

username

If you want to match all the values (websiteName, userAgentNameWithSpaces and so on), you could do the following:

Pattern pattern = Pattern.compile("Rendering content from (.*) using user agent (.*) ; for user (.*) ; at time (.*)");
Matcher matcher = pattern.matcher(s);
if (matcher.find()) {
    System.out.println(matcher.group(1));
    System.out.println(matcher.group(2));
    System.out.println(matcher.group(3));
    System.out.println(matcher.group(4));
}

The output will be:

websiteNAme
userAgentNameWithSpaces
username
someTime

Note that if userAgentNameWithSpaces contains spaces, \w+ won't work (because \w doesn't match spaces), so .* will work in this case.

But you can also use [\w ]+ - the brackes [] means "any of the characters inside me", so [\w ] means "a word character, or a space" (note that there's a space between w and ]. So the code would be (testing with a username with spaces):

String s = "Rendering content from websiteNAme using user agent userAgent Name WithSpaces ; for user username ; at time someTime";
Pattern pattern = Pattern.compile("Rendering content from (.*) using user agent ([\\w ]+) ; for user (.*) ; at time (.*)");
Matcher matcher = pattern.matcher(s);
if (matcher.find()) {
    System.out.println(matcher.group(1));
    System.out.println(matcher.group(2));
    System.out.println(matcher.group(3));
    System.out.println(matcher.group(4));
}

And the output will be:

websiteNAme
userAgent Name WithSpaces
username
someTime

Note: you can test if the groups were matched before calling matcher.group(n). The method matcher.groupCount() returns how many groups were matched (because if you call matcher.group(n) and group n is not available, you'll get an IndexOutOfBoundsException)

edited Apr 20, 2017 at 12:08

answered Apr 19, 2017 at 19:36

user7605325

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2 Comments

Vivek Kumar Over a year ago

Thanks for the explanation. Now, I am using this expression to match userAgentWithSpaces. Lemme know if this is right. Pattern.compile("using user agent ([\\w*\\s*]*)");

Vivek Kumar Over a year ago

How about this. Pattern TEMPLATE_LOG_PATTERN = Pattern.compile( "Rendering content from (.*)using user agent (.*) ; for user (.*) ; at time (.*)$"); time = System.out.println(matcher.group(4).trim()); userName = System.out.println(matcher.group(3).trim());

brso05 · Accepted Answer · 2017-04-19 19:30:38Z

2

I think you want to use lookaheads and lookbehinds:

String s = "Rendering content from websiteNAme using user agent userAgentNameWithSpaces ; for user username ; at time someTime";
Pattern USER_NAME_PATTERN = Pattern.compile("(?<=for user).*?(?=;)");
final Matcher matcher = USER_NAME_PATTERN.matcher(s);
matcher.find();
System.out.println(matcher.group(0).trim());

Output:

username

answered Apr 19, 2017 at 19:30

brso05

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1 Comment

Andrey Over a year ago

You can replace .*? with [^;]* or even [^;]*+ to save some match attempts.

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