Selecting in Regex Python

Question

I'm writing a regex to find code like this:

if condition:True:
    statements

Where the first colon is instead of ==. I've come up with this so far: r"(if|elif) .* : .* :\n\t"
But I want to find a way to select the first : and replace it with ==. The thing is, I cant find a way to use regex to substitute : without inadvertantly a: replacing the whole if condition etc with == by writing a

re.sub(r"(if|elif) .* : .* :\n\t","==",text)

or b: Replacing every colon in my script with ==, which will cause an Error like

NameError: Expected a ':', received '=='

So, is there a way to substitute only a bit of the regex with ==, or is there another way to do this that I overlooked??
What should happen
Input:

if 3+4:7:
     print("Three plus four is actually seven")
elif 3+6:10:
    print("3+4 is not 7, yet 3+6=10")

Console:
Three plus four is actually seven

This is Python 3 by the way.

Sunitha · Accepted Answer · 2018-07-30 12:54:00Z

3

Try matching everything from if or elif upto last-but one : into a match-group and similarly everything after that to the last : into another group. Now you can substitute the first match followed by == and then the next match

re.sub('((?:if|elif)[^:]*):([^:]*:)', r'\1 == \2', s)

edited Jul 30, 2018 at 12:54

answered Jul 30, 2018 at 12:08

Sunitha

12.1k2 gold badges23 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

11 Comments

melwil Over a year ago

Only improvement I can think of is to do [^:]* instead of .*, so it doesn't match more colons than it should in case some line should happen to have 3 (for some other odd reason). Generally one should try to avoid using .* if it's possible.

Sunitha Over a year ago

@melwil. I thought of doing it, but then it would wrongly stop on the colon incase the expression contains colons

melwil Over a year ago

If the expression contains more colons than the example, there's no guarantee this regex would substitute that correctly anyway. My point is that this regex can actually ignore what little pattern you actually have to match on. Using the negated expression brings in another interesting thing with the fact that it matches newlines, though, so you'd need to turn on multiline and match the end of line with $. Your original regex would probably do just fine.

Shanavas M Over a year ago

re.sub('((?:if|elif)[^:]*):([^:]*:)', '\\1 == \\2', text) the last : must be inside the group

melwil Over a year ago

No need for the anchor and flag unless you use the negated colon [^:] group instead of the dot. . does not match newlines without the dotall (s) flag.

|

Collectives™ on Stack Overflow

Selecting in Regex Python

1 Answer 1

11 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

11 Comments

Your Answer

Sign up or log in

Post as a guest

Related