Regular Expressions 101

Explanation

Pro

^start of line

Non-Capturing Group(?:[A-Z0-9]|(?<=[0-9])[a-z])*

*repeats the group contents below any number of times:

1st Alternative[A-Z0-9]

Character Class[A-Z0-9]

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

0-9one character from 0 (48₁₀) to 9 (57₁₀)

2nd Alternative(?<=[0-9])[a-z]

Positive Lookbehind(?<=[0-9])

Character Class[0-9]

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Character Class[a-z]

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

$end of line

Global pattern flags

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

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

Detailed match information will be displayed here automatically.

25 tokens

^(?:[A-Z0-9]|(?<=[0-9])[a-z])*$

New

These are expected matches:
AA
AB
1a
1b
1A
1B
A1A
A1a
AB1
AB2
AB1a
AB2b

These would fail:
Aa
Ab
a1
b1
Ab1
a1a

These are expected matches:
AA
AB
1a
1b
1A
1B
A1A
A1a
AB1
AB2
AB1a
AB2b

These would fail:
Aa
Ab
a1
b1
Ab1
a1a