invalid expression/missing group name

Question

I'm using python 2.7.17 and am trying to do some regular expression manipulation. Everything was working up until I ran into the following error:

Traceback (most recent call last):
  File "latex/latex.py", line 130, in <module>
    contents = re.sub(re.escape(i),img,contents, 0, re.MULTILINE)
  File "/usr/lib/python2.7/re.py", line 155, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/usr/lib/python2.7/re.py", line 286, in _subx
    template = _compile_repl(template, pattern)
  File "/usr/lib/python2.7/re.py", line 273, in _compile_repl
    raise error, v # invalid expression
sre_constants.error: missing group name

I don't know what's wrong because I am actually escaping the string before putting it into sub:

print "----"                                                                   
print i                                                                        
print re.escape(i)                                                             
print img                                                                      
contents = re.sub(re.escape(i),img,contents, 0, re.MULTILINE)

Here is what the print statements produce:

----
$$n \in \Z_{\geq 0}$$
\$\$n\ \\in\ \\Z\_\{\\geq\ 0\}\$\$
<img class="latex inline" src="{filename}/images/latex-cache/symmetric-functions.md827a2321d2328298b1d5789840928039.png" /><!--n \in \Z_{\geq 0}-->

As you can see, it's properly escaping the string, but for some reason it's throwing the error. I don't know what the mistake is because all articles I can find (such as regex error : raise error, v # invalid expression ) don't escape the string.

Any help would be appreciated.

Edit

The contents of content is pulled from a file. I edited the script to also print the contents. Here is what I have in contents:

<div class="content">
Let $$n \in \Z_{\geq 0}$$
</div>

Edit 2

It looks like the error is coming from img. When I replace img with anything else, it works perfectly. I was able to narrow it down to it failing when:

img = "\g"

Am I supposed to be escaping img in some way?

I also tried:

img = "\\g"

and this also caused the same error.

I believe the value of `i` you've shown above is **not** the value that actually caused the error. — Błotosmętek, Mar 21 '20 at 12:11
What's in the variable ```contents``` passed in ```re.sub()``` ? — s.dallapalma, Mar 21 '20 at 13:19
@s.dallapalma I just edited my post with the value of "contents" — Aram Papazian, Mar 21 '20 at 14:34
What do you expect to see in contents after the ```re.sub()```? — s.dallapalma, Mar 21 '20 at 14:39
I cannot find an explanation for a possible GREP code `\g` but still: does the problem magically go away if you properly write "img = '\\g"` instead? (*Theoretically*, any unparsed escaped code is left as it is in the input. Still: since there are lots of escape+one-letter-codes, best always double up your backslashes or use a `r` prefix.) — Jongware, Mar 21 '20 at 15:27
I just tried with "img = "\\g" " as you suggested and it's still giving the same error .-. — Aram Papazian, Mar 21 '20 at 16:51

score 2 · Accepted Answer · answered Mar 21 '20 at 23:03

So it turns out the reason why is because \g is special in re.sub and is used for groups found (same as \1, \2, etc.) So it's a special character. In order to get around that you need to escape it, but since python is fun with escaping you need to escape the escape. In other words you want:

contents = re.sub(re.escape(i),img.replace('\\','\\\\'),contents, 0, re.MULTILINE)

For a more thorough explanation, you can check out the answers in: Python Regex escape operator \ in substitutions & raw strings

invalid expression/missing group name

Edit

Edit 2

1 Answers1