Java String.split() Regex

Question

I have a string:

String str = "a + b - c * d / e < f > g >= h <= i == j";

I want to split the string on all of the operators, but include the operators in the array, so the resulting array looks like:

[a , +,  b , -,  c , *,  d , /,  e , <,  f , >,  g , >=,  h , <=,  i , ==,  j]

I've got this currently:

public static void main(String[] args) {
    String str = "a + b - c * d / e < f > g >= h <= i == j";
    String reg = "((?<=[<=|>=|==|\\+|\\*|\\-|<|>|/|=])|(?=[<=|>=|==|\\+|\\*|\\-|<|>|/|=]))";

    String[] res = str.split(reg);
    System.out.println(Arrays.toString(res));
}

This is pretty close, it gives:

[a , +,  b , -,  c , *,  d , /,  e , <,  f , >,  g , >, =,  h , <, =,  i , =, =,  j]

Is there something I can do to this to make the multiple character operators appear in the array like I want them to?

And as a secondary question that isn't nearly as important, is there a way in the regex to trim the whitespace off from around the letters?

You could just split by spaces in your example expression to get the result you want. — Jeffrey
– Jeffrey, Commented Mar 25, 2012 at 0:31
for your secondary question: String has a trim function: docs.oracle.com/javase/7/docs/api/java/lang/String.html#trim() — ma cılay
– ma cılay, Commented Mar 25, 2012 at 0:32
@Jeffrey: The spaces won't necessarily be there. I have the spaces in there for ease of readability, but it could be any combination of spaces or none. Thanks for the idea though! — user677786
– user677786, Commented Mar 25, 2012 at 0:36
@user306848: Yeah, I know about trim, I was just curious if it was possible in the regex. Thanks for the tip though! — user677786
– user677786, Commented Mar 25, 2012 at 0:37

mmaag · Accepted Answer · 2012-03-25 09:49:31Z

55

String[] ops = str.split("\\s*[a-zA-Z]+\\s*");
String[] notops = str.split("\\s*[^a-zA-Z]+\\s*");
String[] res = new String[ops.length+notops.length-1];
for(int i=0; i<res.length; i++) res[i] = i%2==0 ? notops[i/2] : ops[i/2+1];

This should do it. Everything nicely stored in res.

edited Mar 25, 2012 at 9:49

answered Mar 25, 2012 at 1:44

mmaag

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3 Comments

Chris White Over a year ago

Yeap, this works, just strip off the leading element from the array (which is empty)

user677786 Over a year ago

After coming back, this seems like the best way to do it. I'd like to have done it in the regex, but this will work perfectly. Thanks!

Eggcellentos Over a year ago

Any explanation?

user unknown · Accepted Answer · 2012-03-25 00:43:52Z

20

str.split (" ") 
res27: Array[java.lang.String] = Array(a, +, b, -, c, *, d, /, e, <, f, >, g, >=, h, <=, i, ==, j)

answered Mar 25, 2012 at 0:43

user unknown

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Comments

dat_clover · Accepted Answer · 2014-01-04 07:30:21Z

6

    String str = "a + b - c * d / e < f > g >= h <= i == j";
    String reg = "\\s*[a-zA-Z]+";

    String[] res = str.split(reg);
    for (String out : res) {
        if (!"".equals(out)) {
            System.out.print(out);
        }
    }

Output : + - * / < > >= <= ==

answered Jan 4, 2014 at 7:30

dat_clover

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Comments

Andrew Morton · Accepted Answer · 2012-03-25 00:44:22Z

1

You could split on a word boundary with \b

answered Mar 25, 2012 at 0:44

Andrew Morton

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2 Comments

tchrist Over a year ago

Did you try it? You’re going to have a problem.

Andrew Morton Over a year ago

OK, I admit it, I tested it in .NET and it worked. Removing the empty entries should be trivial, and removing the spaces in the string is surely easily accomplished with a .replaceAll before applying the Regex.

Chris White · Accepted Answer · 2012-03-25 00:43:27Z

0

Can you invert your regex so split by the non operation characters?

String ops[] = string.split("[a-z]")
// ops == [+, -, *, /, <, >, >=, <=, == ]

This obviously doesn't return the variables in the array. Maybe you can interleave two splits (one by the operators, one by the variables)

answered Mar 25, 2012 at 0:43

Chris White

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1 Comment

user677786 Over a year ago

While not the exact solution, it did give me the idea that worked! Thanks! I'll edit the main post for the solution!

sQuirel · Accepted Answer · 2014-04-11 12:40:27Z

-4

You could also do something like:

String str = "a + b - c * d / e < f > g >= h <= i == j";
String[] arr = str.split("(?<=\\G(\\w+(?!\\w+)|==|<=|>=|\\+|/|\\*|-|(<|>)(?!=)))\\s*");

It handles white spaces and words of variable length and produces the array:

[a, +, b, -, c, *, d, /, e, <, f, >, g, >=, h, <=, i, ==, j]

edited Apr 11, 2014 at 12:40

answered Apr 11, 2014 at 12:09

sQuirel

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