Match to string length by using regex in python

Question

Writing python regex for string. I want the string to be at least 1 symbol and max 30. The problem is that im using 3 sub-blocks in regex letters, so there always must be 3 characters long length. Is it possible to add that condition in this regex (1-30 characters length):

regex = re.compile("^[a-zA-Z]+[a-zA-Z0-9\.\-]+[a-zA-Z0-9]$")
r = regex.search(login)

Thank you.

Answer 1

Although it is not clear which 1 or 2 length character strings you want to accept I propose the following regex:

regex = re.compile("^[a-zA-Z][a-zA-Z0-9\.\-]{0,28}[a-zA-Z0-9]$")

As the middle set includes all other this will directly match all words with length 3-30 as you wish.

I hope this regex also matches your 2 length strings (I just assumed that the first character must be a letter), you need to add something (using '|') for single letter matches.

Answer 2

In general, this is difficult and doing some work outside of the RE (as suggested in the comment by M. Buettner) is often required. Your problem is easier because it can be reduced to a pattern with only one repeating element.

You have one or more letters, followed by one or more of (letter, digit, dot, hyphen) followed by a single (letter or digit), right? If so, the repetition of the first group is not needed. Leave off the + to get

r"^[a-zA-Z][a-zA-Z0-9\.\-]+[a-zA-Z0-9]$"

and you will match exactly the same set of strings. Any extra leading letters past the first will be matched in the second group instead of the first.

Now, the only variable portion of your RE is the middle section. To limit the overall length to 30, all you need do is limit that middle portion to 28 characters. Change the + to {1,28} to get:

r"^[a-zA-Z][a-zA-Z0-9\.\-]{1,28}[a-zA-Z0-9]$"

You can read more about Python REs at:

http://docs.python.org/2/library/re.html