Regex group match exactly n times

Question

I have to validate next string format:

text-text-id-text

Separator is character '-'. Third column must always be id. I wrote next regex (in python) which validates string:

import re

s = 'col1-col2-col3-id' # any additional text at the end
                        # is allowed e.g. -col4-col5
print re.match('^(.*-){3}id(-.*)?$', s) # ok 
print re.match('^(.*-){1}id(-.*)?$', s) # still ok, is should not be

I tried adding non-greedy mode, but result is still the same:

^(.*?-){1}id(-.*)?$

What am I missing in my regex? I could just validate string like this:

>>> import re
>>> print re.split('-', 'col1-col2-col3-id')
['col1', 'col2', 'col3', 'id']

And then check if the third element matches id, but I am interested in why does the first regex works as mentioned above.

Answer 1

Your first regex is incorrect because it asserts that id is present after the first three items.
Your second regex matches the string incorrectly because .* matches hyphens as well.

You should use this regex:

/^(?:[^-]+-){2}id/

Here is a regex demo!

And if you feel a need to anchor a regex to the end, use /^(?:[^-]*-){2}id.*$/!

---

As mentioned by Tim Pietzcker, consider asserting id at the end of the item:

/^(?:[^-]+-){2}id(?![^-])/

Here is an UPDATED regex demo!