Python regex with exclusion of a character

Question

Having a list of texts:

l=["SOMETHING","SOME_1","SOM_1"]

I am looking for a regex pattern to match the first string, but not the second one:

• return TRUE if the text starts with 'SOM' , possible some other characters, and no underscores;
• by consequence, return FALSE if the text starts with 'SOM' and contains at least one underscore (somewhere inside the text, it does not matter where that underscore is)

This is what I tried, but does not work:

import re
l=["SOMETHING","SOME_1","SOM_1"]
find_pattern=re.compile("^SOM[A-Z]*[^_]")
for s in l:
    print bool(find_pattern.match(s))

I am expecting:

True
False
False

But I think the multiplication operand is not correct, because I get:

True
True
False

Answer 1

Try this:

find_pattern = re.compile('^SOM[^_]*(?!\_)$')

for s in l:
    print(bool(find_pattern.match(s)))

# True
# False
# False

Sample and explanation here: https://regex101.com/r/DVzoww/1

Answer 2

import re
l=["SOMETHING","SOME_1","SOM_1"]
find_pattern=re.compile("^SOM[^_]*$")
for s in l:
    print bool(find_pattern.match(s))

Prints the expected result:

True
False
False

---

In the pattern, note the importance of the $ anchor. If this was not present, you would receive false positives.