Regular Expressions 101

Explanation
Pro

\ban ASCII word boundary

Positive Lookahead(?=.{5})

.{5}any character (except for line terminators), repeated exactly 5 times

Positive Lookahead(?=(?:[A-Z]\d|\d[A-Z]))

Non-Capturing Group(?:[A-Z]\d|\d[A-Z])

1st Alternative[A-Z]\d

Character Class[A-Z]

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

\da digit

2nd Alternative\d[A-Z]

\da digit

Character Class[A-Z]

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

Group 1(.)

.any character (except for line terminators)

Group 2(.)

.any character (except for line terminators)

Character Class[a-z]

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

\2the text saved by group 2

\1the text saved by group 1

\ban ASCII word boundary

Global pattern flags

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

Detailed match information will be displayed here automatically.

25 tokens

\b(?=.{5})(?=(?:[A-Z]\d|\d[A-Z]))(.)(.)[a-z]\2\1\b

New

K1k1K
aK1k1K
K1k1Ka
aK1k1Ka
bla K1k1K bla