regex101: PCRE (Legacy) Match Workspace

Pro

^start of line

Comment: beginning of string

Non-Capturing Group(?: . # match a single character (?= .*? # if we already have captured the end of the string (?(1) # make sure we do not go past the correct position (?= .\1$ ) ) # capture the end of the string +1 character, adding to \1 every iteration ( .\1?$ ) ) )*

*repeats the group contents below any number of times:

.any character (except for line terminators)

Comment: match a single character

Positive Lookahead(?= .*? # if we already have captured the end of the string (?(1) # make sure we do not go past the correct position (?= .\1$ ) ) # capture the end of the string +1 character, adding to \1 every iteration ( .\1?$ ) )

.*?any character (except for line terminators), repeated any number of times

Comment: if we already have captured the end of the string

Condition(?(1) # make sure we do not go past the correct position (?= .\1$ ) )

Uses the first branch if group 1 matched

If the condition is true, match this branch: # make sure we do not go past the correct position (?= .\1$ )

Comment: make sure we do not go past the correct position

Positive Lookahead(?= .\1$ )

.any character (except for line terminators)

\1the text saved by group 1

$end of line

Comment: capture the end of the string +1 character, adding to \1 every iteration

Group 1( .\1?$ )

.any character (except for line terminators)

\1?optionally matches the text saved by group 1

$end of line

Comment: repeat

Comment: the middle character follows, capture it

Group 2(.)

.any character (except for line terminators)

Global pattern flags

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

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

25 tokens

Regular Expression

^ # beginning of string
(?:
  . # match a single character
  (?=
    .*?
    # if we already have captured the end of the string
    (?(1)
      # make sure we do not go past the correct position
      (?= .\1$ )
    )
    # capture the end of the string +1 character, adding to \1 every iteration
    ( .\1?$ )
  )
)* # repeat
# the middle character follows, capture it
(.)

New

1
123
12345
1234567
123456789
12345678901
1234567890123
123456789012345

Extraction

1
123
12345
1234567
123456789
12345678901
1234567890123
123456789012345