how to replace '+' with 'plus' using regular expression

I'd like to replace

[0-9]+e|E\+[0-9]+ with [0-9]+e|Eplus[0-9]+

and

[0-9]+e|E-[0-9]+ with [0-9]+e|Eminus[0-9]+

I tried this python script:

re.sub(r'[0-9]+[eE]\+[0-9]+$', r'[0-9]+[eE]plus[0-9]+$', 'a1e+1b+a')

I hope to get 'a1eplus1b+a', but it doesn't work. How to do that properly?

The second argument to re.sub is not a regex. You need to use a completely different syntax for the replacement string. In very brief, you can refer back to numbered matching groups with \1, \2, etc;

re.sub(r'([0-9]+[eE])\+([0-9]+)', r'plus', 'a1e+1b+a')

... or, you can use lookarounds to just match and replace the part you really want to replace:

re.sub(r'(?<=[0-9][eE])\+(?=[0-9])', r'plus', 'a1e+1b+a')

The second one also extends nicely to a solution where you can do minus and plus in one fell swoop:

re.sub(r'(?<=[0-9][eE])[-+](?=[0-9])',
       lambda match: {'-': 'minus', '+': 'plus'}[match.group()],
       'a1e+1b+a')

Here, the lambda expression receives the re.Match object corresponding to the match, and use its .group() method to extract the string which matched. We use that as an index into the dictionary of replacements, to obtain the spelled-out string corresponding to either 'plus' or 'minus'.

Python's re module does not allow lookarounds to be variable length, but that's not really useful here anyway, so I just took out the + after [0-9]. Notice also that I took out the $ which prevented your test case from matching.