Regular Expressions 101

^start of line

Group 1((.)(?:(?1)|.?)\2|(.)\3*)

1st Alternative(.)(?:(?1)|.?)\2

Group 2(.)

.any character (except for line terminators)

Non-Capturing Group(?:(?1)|.?)

1st Alternative(?1)

(?1)calls group 1 as a subroutine

2nd Alternative.?

.?optionally matches any character (except for line terminators)

\2the text saved by group 2

2nd Alternative(.)\3*

Group 3(.)

.any character (except for line terminators)

\3*the text saved by group 3, repeated any number of times

$end of line

Global pattern flags

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

g modifier: global. Finds all matches instead of stopping after the first

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

^((.)(?:(?1)|.?)\2|(.)\3*)$

New

a
aa
aba
abba
aaaa
aaaaa
abccba
12344321
0000010000
1234567890123456
1234321
bbbbbbbbbbbbbbb
aaaaaaaaaaa
abcdefghihgfedc
abcba
123431
ab
1232
上海自来水来自海上
ABCBAABCBA