Java Regex does not match

Question

I know that this kind of questions are proposed very often, but I can't figure out why this RegEx does not match. I want to check if there is a "M" at the beginning of the line, or not. Finaly, i want the path at the end of the line. This is why startsWith() doesn't fit my Needs.

line = "M            72208    70779 koj          src\com\company\testproject\TestDomainf1.java";

if (line.matches("^(M?)(.*)$")) {}

I've also tried the other way out:

Pattern p = Pattern.compile("(M?)");
Matcher m = datePatt.matcher(line);
if (m.matches()) {
    System.out.println("yay!");
}

if (line.matches("(M?)(.*)")) {}

Thanks

And what specific misbehavior are you getting? (Also, is your actual scenario more complicated than what you posted? If not, why aren't you just using line.charAt(0)?) — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Aug 19, 2013 at 13:01
M? means "0 or 1 M characters", which will always be true. — jbabey
– jbabey, Commented Aug 19, 2013 at 13:01
what i want is the path at the end of the line. That's why i worked with () — JuergenKie
– JuergenKie, Commented Aug 19, 2013 at 13:20
In this case you need to first find where the path begins. Basically, it's a new question now. — Marko Topolnik
– Marko Topolnik, Commented Aug 19, 2013 at 13:33

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

The correct regex would be simply

line.matches("M.*")

since the matches method enforces that the whole input sequence must match. However, this is such a simple problem that I wonder if you really need a regex for it. A plain

line.startsWith("M")

or

line.length() > 0 && line.charAt(0) == 'M'

or even just

line.indexOf('M') == 0

will work for your requirement.

Performance?

If you are also interested in performance, my second and third options win in that department, whereas the first one may easily be the slowest option: it must first compile the regex, then evaluate it. indexOf has the problem that its worst case is scanning the whole string.

UPDATE

In the meantime you have completely restated your question and made it clear that the regex is what you really need. In this case the following should work:

Matcher m = Pattern.compile("M.*?(\\S+)").matcher(input);
System.out.println(m.matches()? m.group(1) : "no match");

Note, this only works if the path doesn't contain spaces. If it does, then the problem is much harder.

edited Jun 20, 2020 at 9:12

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answered Aug 19, 2013 at 13:03

Marko Topolnik

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3 Comments

cl-r Over a year ago

Performance? : Even if you use static Pattern myPattern = Pattern.compile(regex); for heavy match processes?

JuergenKie Over a year ago

Finally, i want the path at the end of the line. This is why i used () Groups and startsWith() doesn't fit my Needs.

JuergenKie Over a year ago

Did you test the code? If i run it with the input i've posted above, i get no matches at all. It matches when i replace the S with s, but then the Group doesn't contain the path

JuergenKie · Accepted Answer · 2015-08-25 07:15:16Z

3

You dont need a regex for that. Just use String#startsWith(String)

if (line.startsWith("M")) {
    // code here
}

OR else use String#toCharArray():

if (line.length() > 0 && line.toCharArray()[0] == 'M') {
    // code here
}

EDIT: After your edited requirement to get path from input string.

You still can avoid regex and have your code like this:

String path="";
if (line.startsWith("M"))
    path = line.substring(line.lastIndexOf(' ')+1);
System.out.println(path);

OUTPUT:

src\com\company\testproject\TestDomainf1.java

edited Aug 25, 2015 at 7:15

JuergenKie

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answered Aug 19, 2013 at 13:03

anubhava

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Comments

p.s.w.g · Accepted Answer · 2013-08-19 13:09:07Z

3

You can use this pattern to check whether an M character appears as at the beginning of the string:

if (line.matches("M.*"))

But for something this simple, you can just use this:

if (line.length() > 0 && line.charAt(0) == 'M')

edited Aug 19, 2013 at 13:09

answered Aug 19, 2013 at 13:03

p.s.w.g

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4 Comments

Marko Topolnik Over a year ago

The regex is wrong: it will match only the literal line "M". Also, no need for begin/end anchors; that's implied by the semantics of matches.

anubhava Over a year ago

line.matches("^M") will fail if line has any character after M.

Marko Topolnik Over a year ago

And charAt(0) will throw an unwanted exception for empty string.

p.s.w.g Over a year ago

@MarkoTopolnik Thanks, I haven't written any Java in quite a while, so I'm a bit rusty.

Anirudha · Accepted Answer · 2013-08-19 13:03:33Z

0

Why not do this

line.startsWith("M");

answered Aug 19, 2013 at 13:03

Anirudha

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Comments

Prabhakaran Ramaswamy · Accepted Answer · 2013-08-19 13:04:36Z

0

   String str = new String("M 72208 70779 kij src/com/knapp/testproject/TestDomainf1.java");

   if(str.startsWith("M") ){
        ------------------------
        ------------------------
   }

answered Aug 19, 2013 at 13:04

Prabhakaran Ramaswamy

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2 Comments

JuergenKie Over a year ago

Finally, i want the path at the end of the line. This is why startsWith() doesn't fit my Needs.

cl-r Over a year ago

@der_juergen Add an EDIT to your post and complete it with the new answer (do not delete first lines, new readers will understand the thread)

cl-r · Accepted Answer · 2013-08-19 14:05:27Z

0

If you need Path, you can split (I guess than \t is the separator) the string and take the latest field:

String[] tabS = "M            72208    70779 kij          src\com\knapp\testproject\TestDomainf1.java".split("\t");
String path = tabS[tabS.length-1];

edited Aug 19, 2013 at 14:05

answered Aug 19, 2013 at 13:55

cl-r

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