Question

Please consider this sample:

header-1
    item1-1
    item1-2
    item1-3
header-2
    item2-1
    item2-2
...

and I want it in this format:

header-1 item1-1
header-1 item1-2
header-1 item1-3
header-2 item2-1
header-2 item2-2
...

I guess there is easy way to do this with regex, but I just can’t figure out

Any regex syntax is welcomed, I use RegexBuddy under Wine

Answer 1

With RegexBuddy, you can do it in two steps.

First search for (?<=(^\S.*$)(?s:.*?))^\s+ and replace all with \1<space>.

This gives you

header-1
header-1 item1-1
header-1 item1-2
header-1 item1-3
header-2
header-2 item2-1
header-2 item2-2

Explanation:

(?<=   # Make sure we're right after the following match:
 (     # Match and capture in group 1 (the header):
  ^    # From the start of the line...
  \S   # but only if the first character is not whitespace
  .*   # match any number of characters except newlines
  $    # until the end of the line (OK, that's redundant).
 )     # End of group 1
 (?s:  # Start a non-capturing group, DOTALL mode enabled
  .*?  # that matches any number of any character, as few as possible.
 )     # End of group
)      # End of lookbehind assertion
^\s+   # Now match one or more whitespace characters at the start of the line

Then search for ^(.*)$\r?\n(?=\1) and replace with the empty string.

This results in

header-1 item1-1
header-1 item1-2
header-1 item1-3
header-2 item2-1
header-2 item2-2

Explanation:

^       # Match from the start of the line
(.*)    # Match and capture the entire line in group 1
$       # Match until the end of the line (OK, redundant again)
\r?\n   # Match a linebreak
(?=\1)  # Do all this only if the next line starts with the same string as above