Regex匹配线仅当前面没有空行时

您可以替换的(零宽度)匹配项

(?<=(?<!\n)\n)(?=# header$)

具有 "\n" 。

Demo

正则表达式可以分解如下。

(?<=         # begin positive lookbehind
  (?<!       # begin negative lookbehind
    \n       # match literal
  )          # end negative lookbehind
  \n         # match literal
)            # end positive lookbehind
(?=          # begin positive lookahead
  # header   # match literal
  $          # match end of string
)            # end positive lookahead

注意,积极的背后看

(?<=(?<!\n)\n)

具有与(相同的效果,可以替换为)

(?<=\n)(?<!\n\n)

我不确定您要针对哪种语言,但这里有一个Javascript中的例子,使用 String.replace :

let original = 
`a = 1
b = 2

# header
f = 10
g = 20
# header
x = abc
y = def`;

let replaced = original.replace(/(?<!\n\n)(# header)/, '\n$1');
console.log(replaced);

或者在python中来自 Codepen :

import re
s='''a = 1
b = 2

# header
f = 10
g = 20
# header
x = abc
y = def'''
x = re.sub(r'(?<!\n\n)(# header)', r'\n\1', s)
print(x)