Replace spaces with non-breaking spaces according to a specific criterion

I want to clean up files that contain bad formatting, more precisely, replace "normal" spaces with non-breaking spaces according to a given criterion.

For example:

If in a sentence, I have:

"You need to walk 5 km."

I need to replace the space between 5 and km with a non-breaking space.

So far, I have managed to do this:

import os

unites = ['km', 'm', 'cm', 'mm', 'mi', 'yd', 'ft', 'in']

# iterate and read all files in the directory
for file in os.listdir():
    # check if the file is a file
    if os.path.isfile(file):
        # open the file
        with open(file, 'r', encoding='utf-8') as f:
            # read the file
            content = f.read()
            # search for exemple in the file
            for i in unites:
                if i in content:
                    # find the next character after the unit
                    next_char = content[content.find(i) + len(i)]
                    # check if the next character is a space
                    if next_char == ' ':
                        # replace the space with a non-breaking space
                        content = content.replace(i + ' ', i + 'u00A0')

But this replace all the spaces in the document and not the ones that I want.
Can you help me?

---

EDIT

after UlfR’s answer which was very useful and relevant, I would like to push my criteria further and make my "search/replace" more complex.

Now I would like to search for characters before/after a word in order to replace spaces with non-breaking spaces. For example :

• I want to search for the phrase "Can the search be hypothetical?"
  I would like the space between hypothetical and ? to be replaced by a non-breaking space.
• Otherwise also "In the search it is necessary to refer to the "{figure 1.12}"
  I would like the space between {, figure and } to be a non-breaking space but also the space between figure and 1.12 to be a non-breaking space (so all spaces in this case).

I’ve tried to do this :

units = ['km', 'm', 'cm', 'mm', 'mi', 'yd', 'ft', 'in']
units_before_after = ['{']

nbsp = 'u00A0'

rgx = re.sub(r'(bd+)(%s) (%s)b'%(units, units_before_after),r'1%s2'%nbsp,text))

print(rgx)

But I’am having some trouble, do you have any ideas to share ?