RegEx Pattern not matching in JAVA

Question

I want to know why this regex group is not working for me in JAVA? It's throwing exception while finding match of group. I am trying to match the numbers separated by dashes.

Pattern p = Pattern.compile("([0-9]+)-([0-9]+)-([0-9]+)-([0-9]+)-([0-9]+)");
Matcher matcher = p.matcher("1-1-3-1-4");
matcher.group(0); // Exception happens here - java.lang.IllegalStateException: No match found

No. Could you please point, if anything specific to take a look which is causing this issue? — Pratik K. Shah, Jun 12 '17 at 03:05
Using `matcher()` doesn't actually try to match anything. It just sets the matcher up. You have to tell it to try a match with one of the [`Matcher` methods](http://docs.oracle.com/javase/8/docs/api/java/util/regex/Matcher.html). — ajb, Jun 12 '17 at 03:05
@f.khantsis [Have a look at this](https://docs.oracle.com/javase/7/docs/api/java/util/regex/Matcher.html#group(int)) — jack jay, Jun 12 '17 at 03:22

Tim Biegeleisen · Accepted Answer · 2017-06-12T03:12:42.707

4

You need to call Matcher#find() to actually get a match:

Pattern p = Pattern.compile("([0-9]+)-([0-9]+)-([0-9]+)-([0-9]+)-([0-9]+)");
Matcher matcher = p.matcher("1-1-3-1-4");
if (matcher.find()) {
    System.out.println(matcher.group(0))
}

If you were expecting multiple matches, you could use a while loop instead of an if statement.

Also note that you actually have five capture groups in your patterns. Capture groups are denoted by placing a portion of the pattern in parentheses. If you don't intend/need to capture the five separated numbers in your pattern individually, then you can consider telling the regex engine not to capture them, e.g. use this:

Pattern p = Pattern.compile("(?:[0-9]+)-(?:[0-9]+)-(?:[0-9]+)-(?:[0-9]+)-(?:[0-9]+)");

Demo

edited Jun 12 '17 at 03:12

answered Jun 12 '17 at 03:07

Tim Biegeleisen

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It should match this pattern but, right ? – Pratik K. Shah Jun 12 '17 at 03:07
Group zero matches the entire pattern. On top of this, you have five capture groups in there. If you can tell us more about what you are trying to do here, I can update my answer. – Tim Biegeleisen Jun 12 '17 at 03:09
Okey, I got it, after using matcher.find() it worked for me. Thanks :) – Pratik K. Shah Jun 12 '17 at 03:11

score 1 · Answer 2 · answered Jun 12 '17 at 03:25

In Java regex index or study methods are used to return Matcher class matches:

if (matcher.matches()) {
    System.out.println(matcher.group(0));
}

In the example above, the matches() "study" method attempts to match the entire region against the given pattern. Which method you use is generally indicative of what/how you want to match.

matches()

Attempts to match the entire region against the pattern.

find()

Attempts to find the next subsequence of the input sequence that matches the pattern.

_{Study methods review the input string and return a Boolean indicating whether or not the pattern is found}

↳ http://docs.oracle.com/javase/tutorial/essential/regex/matcher.html

RegEx Pattern not matching in JAVA

2 Answers2

Demo