Develop Reference

Java - regular expression to check if begins and ends with certain characters

Considering a string in following format,
[ABCD:defg] [MSG:information] [MSG2:hello]
How to write regex to check if the line has '[MSG:' followed by some message & ']' and extract text 'information' from above string?

You can use the regex, \[MSG:(.*?)\] and extract the value of group(1).
Demo:
import java.util.regex.Matcher;
import java.util.regex.Pattern;
class Main {
public static void main(String args[]) {
String str = "[ABCD:defg] [MSG:information] [MSG2:hello]";
Matcher matcher = Pattern.compile("\\[MSG:(.*?)\\]").matcher(str);
if (matcher.find())
System.out.println(matcher.group(1));
}
}
Output:
information

Your requirement would be something like
/\[MSG:.+\]/ in standard regex notation. But I would suggest to you that you could use String.indexOf to extract your information
String str = ...
int idx = str.indexOf("MSG:");
int idx2 = str.indexOf("]", idx);
val = str.substring(idx + "MSG:".length(), idx2);

Regex to split the first from a "/token1/token2/token3"

I'm pretty rusty with regex, but I have the requirement to extract the first token of the following string:
Input: /token1/token2/token3
Required output: /token1
I have tried:
List<String> connectorPath = Splitter.on("^[/\\w+]+")
.trimResults()
.splitToList(actionPath);
Doesn't work for me, any ideas?

Instead of split, you can match
^/\\w+
Or if the string has 3 parts, use a capture group for the first part.
^(/\\w+)/\\w+/\\w+$
Java example
Pattern pattern = Pattern.compile("^/\\w+");
Matcher matcher = pattern.matcher("/token1/token2/token3");
if (matcher.find()) {
System.out.println(matcher.group(0));
}
Output
/token1

You can split on the / that is not at the string start using the (?!^)/ regex:
String[] res = "/token1/token2/token3".split("(?!^)/");
System.out.println(res[0]); // => /token1
See the Java code demo and the regex demo.
(?!^) - a negative lookahead that matches a location not at the start of string
/ - a / char.
Using Guava:
Splitter splitter = Splitter.onPattern("(?!^)/").trimResults();
Iterable<String> iterable = splitter.split(actionPath);
String first = Iterables.getFirst(iterable, "");

You are over-complicating it.
Try the following regular expression: ^(\/\w+)(.+)$
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class PathSplitter {
public static void main(String args[]) {
String input = "/token1/token2/token3";
Pattern pattern = Pattern.compile("^(\\/\\w+)(.+)$");
Matcher matcher = pattern.matcher(input);
if (matcher.find()) {
System.out.println(matcher.group(1)); // /token1
System.out.println(matcher.group(2)); // /token2/token3
} else {
System.out.println("NO MATCH");
}
}
}

How to preserve delimeters while using String.split() in Java?

String TextValue = "hello{MyVar} Discover {MyVar2} {MyVar3}";
String[] splitString = TextValue.split("\\{*\\}");
What I'm getting output is [{MyVar, {MyVar2, {MyVar3] in splitString
But my requirement is to preserve those delimiters {} i.e. [{MyVar}, {MyVar2}, {MyVar3}].
Required a way to match above output.

Use something like so:
Pattern p = Pattern.compile("(\\{\\w+\\})");
String str = ...
Matcher m = p.matcher(str);
while(m.find())
System.out.println(m.group(1));
Note, the code above is untested but that will look for words within curly brackets and place them in a group. It will then go over the string and output any string which matches the expression above.
An example of the regular expression is available here.

Thanks kelvin & npinti.
import java.util.regex.Pattern;
import java.util.regex.Matcher;
public class CreateMatcherExample {
public static void main(String[] args) {
String TextValue = "hello{MyVar} Discover {My_Var2} {My_Var3}";
String patternString = "\\{\\w+\\}";
Pattern pattern = Pattern.compile(patternString);
Matcher matcher = pattern.matcher(TextValue);
while(matcher.find()) {
System.out.println(matcher.group());
}
}
}

How to replace tokens in java using regex?

I am having a string template containing $variables which needs to be replaced.
String Template: "hi my name is $name.\nI am $age old. I am $sex"
The solution which i tried verifying does not work in the java program.
http://regexr.com/3dtq1
Further, I referred to https://www.regex101.com/ where i could not check if the pattern works for java. But, while going through one of the tutorials I found that "$ Matches end of line". what's the best way to replace the tokens in the template with the variables?
import java.util.HashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class PatternCompiler {
static String text = "hi my name is $name.\nI am $age old. I am $sex";
static Map<String,String> replacements = new HashMap<String,String>();
static Pattern pattern = Pattern.compile("\\$\\w+");
static Matcher matcher = pattern.matcher(text);
public static void main(String[] args) {
replacements.put("name", "kumar");
replacements.put("age", "26");
replacements.put("sex", "male");
StringBuffer buffer = new StringBuffer();
while (matcher.find()) {
String replacement = replacements.get(matcher.group(1));
if (replacement != null) {
// matcher.appendReplacement(buffer, replacement);
// see comment
matcher.appendReplacement(buffer, "");
buffer.append(replacement);
}
}
matcher.appendTail(buffer);
System.out.println(buffer.toString());
}
}

You are using matcher.group(1) but you didn't define any group in the regexp (( )), so you can use only group() for the whole matched string, which is what you want.
Replace line:
String replacement = replacements.get(matcher.group(1));
With:
String replacement = replacements.get(matcher.group().substring(1));
Notice the substring, your map contains only words, but matcher will match also $, so you need to search in map for "$age".substring(1)" but do replacement on the whole $age.

You can try replacing the pattern string with
\\$(\\w+)
and the variable replacement works. Your current pattern only has group 0 (the entire pattern) but not group 1. Adding the parenthesis makes the first group the variable name and the replacement will replace the dollar sign and the variable name.

Your code has just minor glitches.
static Map<String,String> replacements = new HashMap<>();
static Pattern pattern = Pattern.compile("\\$\\w+\\b"); // \b not really needed
// As no braces (...) there is no group(1)
String replacement = replacements.get(matcher.group());

Your not using the right thing as your key. Change to group(), and change map to '$name' etc:
import java.util.HashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class HelloWorld {
static String text = "hi my name is $name.\nI am $age old. I am $sex";
static Map<String,String> replacements = new HashMap<String,String>();
static Pattern pattern = Pattern.compile("\\$\\w+");
static Matcher matcher = pattern.matcher(text);
public static void main(String[] args) {
replacements.put("$name", "kumar");
replacements.put("$age", "26");
replacements.put("$sex", "male");
StringBuffer buffer = new StringBuffer();
while (matcher.find()) {
String replacement = replacements.get(matcher.group());
System.out.println(replacement);
if (replacement != null) {
// matcher.appendReplacement(buffer, replacement);
// see comment
matcher.appendReplacement(buffer, "");
buffer.append(replacement);
}
}
matcher.appendTail(buffer);
System.out.println(buffer.toString());
}
}

Extract part of a string from a URL - Java Regex

I'm trying to extract a string between '/' and '.' of a URL. For example, I have a URL like "some.com/part1/part2/part3/stringINeed.xyz". I need to extract "stringINeed" from the above URL, the one between last '/' and the '.' nothing else.
So far, I tried the following and it gives an empty output:
import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
class Extract
{
public static void main (String[] args) throws java.lang.Exception
{
String str = "part1/part2/part3/stringINeed.xyz" ;
Pattern pattern = Pattern.compile("/(.*?).");
Matcher matcher = pattern.matcher(str);
if (matcher.find()) {
System.out.println(matcher.group(1));
}
}
}
What is wrong with my code. Can anyone help?

Use this regex:
[^/.]+(?=\.[^.]+$)
See demo.
In Java:
Pattern regex = Pattern.compile("[^/.]+(?=\\.[^.]+$)");
Matcher regexMatcher = regex.matcher(subjectString);
if (regexMatcher.find()) {
ResultString = regexMatcher.group();
}
Explanation
[^/.]+ matches any chars that are not a slash or a dot
The lookahead (?=\.[^.]+) asserts that what follows is a dot followed by non-dots and the end of the string

Without regex
str.substring(str.lastIndexOf("/"), str.lastIndexOf(".")).replaceAll("/", "");