regex: replace hyphens with en-dashes with re.sub

Question

I am using a small function to loop over files so that any hyphens - get replaced by en-dashes – (alt + 0150).

The function I use adds some regex flavor to a solution in a related problem (how to replace a character INSIDE the text content of many files automatically?)

def mychanger(fileName):
  with open(fileName,'r') as file:
    str = file.read()
    str = str.decode("utf-8")
    str = re.sub(r"[^{]{1,4}(-)","–", str).encode("utf-8")
  with open(fileName,'wb') as file:
    file.write(str)

I used the regular expression [^{]{1,4}(-) because the search is actually performed on latex regression tables and I only want to replace the hyphens that occur around numbers.

To be clear: I want to replace all hyphens EXCEPT in cases where we have genuine latex code such as \cmidrule(lr){2-4}.

In this case there is a { close (within 3-4 characters max) to the hyphen and to the left of it. Of course, this hyphen should not be changed into an en-dash otherwise the latex code will break.
I think the left part condition of the exclusion is important to write the correct exception in regex. Indeed, in a regression table you can have things like -0.062\sym{***} (that is, a { on the close right of the hyphen) and in that case I do want to replace the hyphen.

A typical line in my table is

variable    &   -2.061\sym{***}&       4.032\sym{**}   &       1.236         \
            &      (-2.32)         &   (-2.02)         &      (-0.14)

However, my regex does not appear to be correct. For instance, a (-1.2) will be replaced as –1.2, dropping the parenthesis.

What is the problem here? Thanks!

regex: replace hyphens with en-dashes with re.sub

2 Answers