Java Regex : Replace characters between two specific characters with equal number of another character

Question

Replacing all characters between starting character "+" and end character "+" with the equal number of "-" characters.
My specific situation is as follows:

Input: +-+-+-+
Output: +-----+

String s = = "+-+-+-+";
s = s.replaceAll("-\\+-","---")

This is not working. How can I achieve the output in Java? Thanks in advance.

You should include how it's not working. What you get instead. — Scratte
– Scratte, Commented Sep 25, 2021 at 9:09
If my answer did not solve your issue please consider updating the question. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Nov 28, 2021 at 19:47

anubhava · Accepted Answer · 2021-09-25 07:49:29Z

1

You can use this replacement using look around assertions:

String repl = s.replaceAll("(?<=-)\\+(?=-)", "-");
//=> +-----+

RegEx Demo

(?<=-)\\+(?=-) will match a + if it is surrounded by - on both sides. Since we are using lookbehind and lookahead therefore we are not consuming characters, these are only assertions.

answered Sep 25, 2021 at 7:49

anubhava

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Comments

Wiktor Stribiżew · Accepted Answer · 2021-09-25 09:25:18Z

0

The matches you have are overlapping, look:

+-+-+-+
 ^^^
    Match found and replaced by "---"

+---+-+
    ^
    +-- Next match search continues from here

WARNING: No more matches found!

To make sure there is a hyphen free for the next match, you need to wrap the trailing - with a positive lookahead and use -- as replacement pattern:

String s = = "+-+-+-+";
s = s.replaceAll("-\\+(?=-)","--")

See the regex demo.

edited Sep 25, 2021 at 9:25

answered Sep 25, 2021 at 9:10

Wiktor Stribiżew

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2 Comments

Wiktor Stribiżew Over a year ago

FYI: Here is a link to the regex debugger. If anyone wants to see how matches are now found in the current string, please click ▶️ button.

Scratte Over a year ago

I tried to put it into a snippet, which works. But it requires the user to both expand and run it.

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