regex: matching a repeating sequence

Question

I'm trying to construct a regular expression that will match a repeating DNA sequence of 2 characters. These characters can be the same.

The regex should match a repeating sequence of 2 characters at least 3 times and, here are some examples:

regex should match on:

• ATATAT
• GAGAGAGA
• CCCCCC

and should not match on:

• ACAC
• ACGTACGT

So far I've come up with the following regular expressions:

[ACGT]{2}

this captures any sequence consisting of exactly two characters (A, C, G or T). Now I want to repeat this pattern at least three times, so I tried the following regular expressions:

[ACGT]{2}{3,}
([ACGT]{2}){3,}

Unfortunately, the first one raises a 'multiple repeat' error (Python), while the second one will simply match any sequence with 6 characters consisting of A, C, G and T.

Is there anyone that can help me out with this regular expression? Thanks in advance.

Answer 1

You could perhaps make use of backreferences.

([ATGC]{2})\1{2,}

\1 is the backreference referring to the first capture group and will be what you have captured.

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