asked on Understanding Greedy Quantifier * and +
I ran these arguments given below:
[wow]* "wow its cool"
Output: 0 "wow" 3 "" 4 "" 5 "" 6 "" 7 "" 8 "" 9 "oo" 11 "" 12 ""
[wow]+ "wow its cool"
Output: 0 "wow" 9 "oo"
I know that * is a greedy quantifier which matches as many as it can and must be zero or more, so there are empty strings after "wow". But can you tell me the reason I got "oo" in the output for both + and *?
I find greedy quantifiers hard to understand. I know + means 1 or more, and * mean zero or more, but I am not sure under what situation I am supposed to use each of them?
Can you simplify the difference between them? (Any analogy or examples would helpful)
I used the code below to try those greedy quantifiers:
I will appreciate your response.
[wow]* "wow its cool"
Output: 0 "wow" 3 "" 4 "" 5 "" 6 "" 7 "" 8 "" 9 "oo" 11 "" 12 ""
[wow]+ "wow its cool"
Output: 0 "wow" 9 "oo"
I know that * is a greedy quantifier which matches as many as it can and must be zero or more, so there are empty strings after "wow". But can you tell me the reason I got "oo" in the output for both + and *?
I find greedy quantifiers hard to understand. I know + means 1 or more, and * mean zero or more, but I am not sure under what situation I am supposed to use each of them?
Can you simplify the difference between them? (Any analogy or examples would helpful)
I used the code below to try those greedy quantifiers:
import java.util.regex.*;
public class TestClass
{
public static void main(String[] args)
{
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
boolean b = false;
while(b = m.find())
{
System.out.print(m.start()+" \""+m.group()+"\" ");
}
}
}
I will appreciate your response.
Java
Last Comment
techbro
* means zero so even when nothing is in between - will matcvh, and + requoires sometihing in between
so "C*L" pattern wll match "CL"
but "C+L" pattern should not match string "CL"
but "C+L" pattern should not match string "CL"
soe [wow]* means that you are looking for the string which is made up of charcaters of "w" or "o" - any number of them,
therefore "oo" should match and "ww" should match or "wo" should match, it probably is not necessary to repeat "w" two times
inside the brackets
therefore "oo" should match and "ww" should match or "wo" should match, it probably is not necessary to repeat "w" two times
inside the brackets
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Ok, now I tested some of my statements above:
"C[wow]*L" should match string "CL" as even nothing between "CL" will match - correct, see below
"C[wow]*L" should match string "CL" as even nothing between "CL" will match - correct, see below
Pattern p6 = Pattern.compile("C[wow]*L");
Matcher m6 = p6.matcher("CL");
boolean b6 = false;
while(b6 = m6.find())
{
System.out.print("output: " + m6.start()+" \""+m6.group()+"\" ");
}
System.out.println("");
output: 0 "CL"
"C[wow]+L" then it should not match string "CL", as it needs more than zero ofg somthing inside the [] - correct, see below
Pattern p6 = Pattern.compile("C[wow]+L");
Matcher m6 = p6.matcher("CL");
boolean b6 = false;
while(b6 = m6.find())
{
System.out.print("output: " + m6.start()+" \""+m6.group()+"\" ");
}
System.out.println("");
No output generated as expected
More tests consistent with the statements above
I guess , all seems understandable.
Please, let me know if you still have any doubts.
code:
output:
code:
output:
I guess , all seems understandable.
Please, let me know if you still have any doubts.
code:
Pattern p6 = Pattern.compile("[wowwowow]+");
Matcher m6 = p6.matcher("oo");
boolean b6 = false;
while(b6 = m6.find())
{
System.out.print("output: " + m6.start()+" \""+m6.group()+"\" ");
}
System.out.println("");
output:
output: 0 "oo"
code:
Pattern p6 = Pattern.compile("[o]+");
Matcher m6 = p6.matcher("oo");
boolean b6 = false;
while(b6 = m6.find())
{
System.out.print("output: " + m6.start()+" \""+m6.group()+"\" ");
}
System.out.println("");
output:
output: 0 "oo"
> But can you tell me the reason I got "oo" in the output for both + and *?
[wow]*
matches 0 or more 'w' or 'o''s.
The other w is actually redundant
Get rid of the square brackets if you want it to just match 'wow'
[wow]*
matches 0 or more 'w' or 'o''s.
The other w is actually redundant
Get rid of the square brackets if you want it to just match 'wow'
As "[wow]*" does not need any single instance to match,
that's why it matches all these empty strings - frankly don't understand how many
empty strings it matches - I guess, one after each character with the exception of matches
that's why it matches all these empty strings - frankly don't understand how many
empty strings it matches - I guess, one after each character with the exception of matches
> but I am not sure under what situation I am supposed to use each of them?
typically when at least a single instance is required then use +
If its optional then *
typically when at least a single instance is required then use +
If its optional then *
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the example in your question also shows the difference.
When you use the + at least one character is required so the empty strings no longer match
When you use the + at least one character is required so the empty strings no longer match
ASKER
Thanks, it makes sense!
matcgh - makes sense