Question

I need to process lines having a syntax similar to markdown http://daringfireball.net/projects/markdown/syntax, where header lines in my case are something like:

=== a sample header ===
===== a deeper header =====

and I need to change their depth, i.e. reduce it (or increase it) so:

== a sample header ==
==== a deeper header ====

my small knowledge of python regexes is not enough to understand how to replace a number
n of ‘=’ ‘s with (n-1) ‘=’ signs

Answer 1

You could use backreferences and two negative lookarounds to find two corresponding sets of = characters.

output = re.sub(r'(?<!=)=(=+)(.*?)=\1(?!=)', r'\1\2\1', input)

That will also work if you have a longer string that contains multiple headers (and will change all of them).

What does the regex do?

(?<!=)  # make sure there is no preceding =
=       # match a literal =
(       # start capturing group 1
  =+    # match one or more =
)       # end capturing group 1
(       # start capturing group 2
  .*?   # match zero or more characters, but as few as possible (due to ?)
)       # end capturing group 2
=       # match a =
\1      # match exactly what was matched with group 1 (i.e. the same amount of =)
(?!=)   # make sure there is no trailing =