Escaping dollar signs in regex not working
python, regex
Solution
The reason your current regex isn't working is that you have a `\b` just before your class name. `\b` will match word boundaries, so only between word characters and non-word characters. For the string `foo = Bar()`, the `\b` will match between the space and the `B`, but for `foo = $Bar()`, the `\b` cannot match between the space and the `$` because they are both non-word characters.
To fix this, change `\b` to `(?=\b|\B\$)`, here is the resulting regex:
for classname in allowed_classes:
line = re.sub(r'^([^\'"]*(?:([\'"])[^\'"]*\2)*[^\'"]*)(?=\b|\B\$)(%s\s*\()' % classname, r'\1new \3', line)
By using a lookahead, you can handle both of the following cases:
- `classname` does not start with `$`, so we want a word boundary before trying to match `classname`, the `\b` inside of the lookahead handles this
- `classname` does start with `$`, so if the next character is a `$` we want to match. I used `\B\$` so it will only match if the character before the `$` is not a word character, but this is probably unnecessary since I can't think of any valid JS code where that would be the case
Problem
Before I start, I know there are better ways than regex doing this (like tokenizers), that's not what the question is about. I'm already stuck using regex, and it already works as I need to, except one special case, which is what I need advice on. I need to scan through some JavaScript-like code and insert the `new` keyword in front of every object declaration. I already know the names of all objects that will need this keyword, and I know that none of them will have that keyword in the code before I start (so I don't need to deal with repeated `new` words or guessing whether something is an object or not. For example, a typical line could look like this: ``` foo = Bar() ``` Where I would already know that `Bar` is a 'class' and would need 'new' for object declaration. The following regex does the trick: ``` for classname in allowed_classes: line = re.sub(r'^([^\'"]*(?:([\'"])[^\'"]*\2)*[^\'"]*)\b(%s\s*\()' % classname, r'\1new \3', line) ``` It works like a charm, even making sure not to touch `classname` when it's inside a string (The first portion of the regex tells it to make sure there are even number of quotes before-hand - it's a bit naive in that it will break with nested quotes, but I don't need to handle that case). Problem is, class names could also have `$` in them. So the following line is allowed as well if `$Bar` exists in allowed_classes: ``` foo = $Bar() ``` The above regex will ignore it, due to the dollar sign. I figured escaping it would do the trick, but this logic seems to have no effect on the above line even if `$Bar` is one of the classes: ``` for classname in allowed_classes: line = re.sub(r'^([^\'"]*(?:([\'"])[^\'"]*\2)*[^\'"]*)\b(%s\s*\()' % re.escape(classname), r'\1new \3', line) ``` I also tried escaping it by hand using `\` but it has no effect either. Can someone explain why converting `$` to `\$` isn't working and what could fix it? Thanks