Hi all!

I am trying to understand the following regular expression.

In the above figure the author have used regular expression to identify G4 motifs in greedy and non-greedy fashion.

• Non-overlapping Greedy (?:(G{3}[ATGC]{1,7}){3}G{3})
• Non-overlapping Non-Greedy (G{3}[ATGC]{1,7}){3}G{3}
• overlapping Greedy (?=((G{3}[ATGC]{1,7}){3}G{3}))
• overlapping non-Greedy (?=((?:(G{3}ATGC{0,6}){3}G{3})))

However I am unable to understand the utility of ?: and ?= and combination thereof (used in last regular expression). Can anyone help? I used the regex