仅删除某些其他匹配行之间的空白行

Question

I am trying to remove blank lines between other lines that match a particular pattern. In my case, that pattern is just that the line begins with a - character.

const orig = `
- line1

- line2

- line3

- line4

- line5
`.trim();

const actual =
  orig.replace(/((?:^|\n)-.*\n)\n(-)/g, '$1$2');

In the code above, I'm using a regex to match:

• a newline (or string start), followed by...
• a - prefixed line, followed by..
• an empty line, followed by...
• another -

I'm globally replacing the entire expression with the two capture groups that omit the empty line between them. This sort of works like I expected it to, but omits every other empty line, and I don't know why.

Where I would have expected the code above to give me this:

- line1
- line2
- line3
- line4
- line5

...it actually gives me this:

- line1
- line2

- line3
- line4

- line5

Here is a fiddle that demonstrates the problem.

Question: What about the regex is causing this behavior?

Bonus: Is there a better way to do this? (eg via split / reduce - although I would still like to know why it doesn't work)

Answer 1

The last - is a part of the consuming pattern. Once the (-) matches, the regex index is set after that - , and you cannot find that match as - in (?:^|\\n)- cannot match that - . You need to put it into a positive lookahead. Then, you need to use m modifier to let ^ match start of a line positions, not just start of string.

Use

/((?:^|\n)-.*\n)\n(?=-)/gm

See the regex demo . Replacement string is reduced to $1 since there is only one capturing group left.

Here is the fixed expression demo:

 const orig = ` - line1 - line2 - line3 - line4 - line5 `.trim(); const actual = orig.replace(/((?:^|\\n)-.*\\n)\\n(?=-)/gm, '$1'); document.getElementById('orig').innerText = orig; document.getElementById('actual').innerText = actual; 

 ul { font-family: sans-serif; list-style: none; padding: 0; } li { display: inline-block; padding: 1em; vertical-align: top; } 

 <ul> <li><h3>Original</h3><pre id="orig"></pre></li> <li><h3>Expected</h3><pre>- line1<br />- line2<br />- line3<br />- line4<br />- line5</pre></li> <li><h3>Actual</h3><pre id="actual"></pre></li> </ul> 

Answer 2

The reason for this behavior is that the regex does not overlap matches. It consumes and matches:

- line 1

- 

Replaces with:

- line 1
- 

And then continues traversing the string from the end of its previous match.

For this reason it does not match the next newline, because

  line 2

- line 3

Does not contain a match your pattern. The next match to your pattern will be

<newline>
- line 3

-

Replaced by:

<newline>
- line 3
-

A way to solve this is by using either lookaheads or lookbehinds , which allow conditional matching based on surrounding patterns without consuming those patterns .

We can modify your pattern slightly to use a lookahead to make sure the next line adheres to the pattern

const actual = orig.replace(/^(-.*\n)\n(?=-)/gm, '$1');

https://regex101.com/r/fPUkYh/4

I also changed ((?:^|\\n)-.*\\n)\\n to ^(-.*\\n)\\n and added the m flag because the start of line assertion ^ does not need to be in the capturing group and the \\n leads to the removal of preceding newlines.

This pattern could also be modified to match an arbitrary number of bl;ank lines in between lines matching the pattern:

/^(-.*\n)\n+(?=-)/gm

https://regex101.com/r/X7B7pi/2

Answer 3

Easy enough when using the Multi-line modifier //m

 (                             # (1 start), Stuff to write back
      ^                             # BOL
      - .* 
      \r? \n      
 )                             # (1 end)
 \s*                           # Blank lines to remove
 \r? \n 

 var orig_str = "- line1\\n\\n\\n- line2\\n\\n- line3\\n\\n- line4\\n\\n- line5\\n- line6"; var new_str = orig_str.replace(/(^-.*\\r?\\n)\\s*\\r?\\n/mg, '$1'); console.log( "Original\\n--------\\n" + orig_str + "\\n" ); console.log( "New\\n--------\\n" + new_str ); 

Output

Original
--------
- line1


- line2

- line3

- line4

- line5
- line6


New
--------
- line1
- line2
- line3
- line4
- line5
- line6

---

If just between -lines is what you need, just add an assertion at the
end (^-.*\\r?\\n)\\s*\\r?\\n(?=-)

Answer 4

You can do it in the following way

 const orig = ` - line1 - line2 - line3 - line4 - line5 `.trim(); const actual = orig.replace(/(\\-[^\\n]*)([^-]*)(?=-)/g, '$1\\n'); document.getElementById('orig').innerText = orig; document.getElementById('actual').innerText = actual; 

 <ul> <li><h3>Original</h3><pre id="orig"></pre></li> <li><h3>Expected</h3><pre>- line1<br />- line2<br />- line3<br />- line4<br />- line5</pre></li> <li><h3>Actual</h3><pre id="actual"></pre></li> </ul> 

see the regex demo

Answer 5

这里是一个较短的正则表达式，包括您要进行加工的模式：

const actual = orig.replace(/(-.*\\n)\\n/g, '$1');

Answer 6

这会给您您所需要的-

const actual = orig.replace(/\n\n|\r\r/g, "\n");