Java正则表达式未捕获组

Question

I am applying a regex to the following string:

2x#0$×4x#3$÷5x#0$

using the following code:

String monomialRegex = "(-?\\d+(\\.\\d*)?[x][#]-?\\d+(\\.\\d*)?[\\$])";
String string2 = "2x#0$×4x#3$÷5x#0$";//the × between $ and 4 is a times symbol
string2 = string2.replaceAll(monomialRegex+"([×])"+monomialRegex+"([÷])"+monomialRegex,"÷$1%$5@×$3");

my result is coming out as: ÷2x#0$%4x#3$@×

From what I understand about groups:

$1 = 2x#0$
$2 = ×
$3 = 4x#3$
$4 = ÷
$5 = 5x#0$

in the result, I see that instead of using group $5 it uses group $3 AND group $3 does not even appear?

Any advice as to how to handle this regex expression?

Answer 1

Your full regex is :

(-?\d+(\.\d*)?[x][#]-?\d+(\.\d*)?[\$])([×])(-?\d+(\.\d*)?[x][#]-?\d+(\.\d*)?[\$])([÷])(-?\d+(\.\d*)?[x][#]-?\d+(\.\d*)?[\$])

Its capturing groups are :

1. (-?\\d+(\\.\\d*)?[x][#]-?\\d+(\\.\\d*)?[\\$])
2. (\\.\\d*)
3. (\\.\\d*)
4. ([×])
5. (-?\\d+(\\.\\d*)?[x][#]-?\\d+(\\.\\d*)?[\\$])
6. (\\.\\d*)
7. (\\.\\d*)
8. ([÷])
9. (-?\\d+(\\.\\d*)?[x][#]-?\\d+(\\.\\d*)?[\\$])
10. (\\.\\d*)
11. (\\.\\d*)

With your current input, the following groups have content :

• 1 : 2x#0$
• 4 : ×
• 5 : 4x#3$
• 8 : ÷
• 9 : 5x#0$

You can visualize that on regex101 : https://regex101.com/r/Nh0xxW/1

I would use the following to accomplish the same goal (at least if I've guessed it correctly) :

Match (-?\d+(?:\.\d*)?x#-?\d+(?:\.\d*)?\$)×(-?\d+(?:\.\d*)?x#-?\d+(?:\.\d*)?\$)÷(-?\d+(?:\.\d*)?x#-?\d+(?:\.\d*)?\$)
Replace by %$1%$2@×$3

As seen here : https://regex101.com/r/Nh0xxW/2

I mainly simplified the regex by removing useless groups (no need to capture decimals which are part of the whole monomial capturing group nor ÷ and × which you don't use in the replacement) and single-character character classes.

You can still craft the regex with concatenation of a more simple one if you want (although I find it makes the whole thing more complex to understand in the end), and the following code will do so :

String monomialRegex = "(-?\\d+(?:\\.\\d*)?x#-?\\d+(?:\\.\\d*)?\\$)";
String completeRegex = monomialRegex + "×" + monomialRegex + "÷" + monomialRegex;

I've tested the java code on ideone : https://ideone.com/Eg8Zz6

Answer 2

The problem was in the sub-groups inside monomialRegex:

I got rid of the subgroup: (\\.\\d*)? within monomialRegex.

I replaced this subgroup with [\\.]?[\\d+]? and it works. I did not need to change the ordering of the groups within the entire regex.