Regular Expressions 101

Explanation
Pro

1st Alternative^([^a-zA-Z]*)([a-z])

^start of line

Group 1([^a-zA-Z]*)

Negated Character Class[^a-zA-Z]*

*matches any number of characters outside the set below:

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

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

Group 2([a-z])

Character Class[a-z]

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

2nd Alternative(\.\s*)([a-z])

Group 3(\.\s*)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\s*any whitespace character, repeated any number of times

Group 4([a-z])

Character Class[a-z]

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

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-zA-Z]*)([a-z])|(\.\s*)([a-z])

New

.this sentence
and this
This is already
.this sentence
and this.also this. and this.This one is not.

$1\U$2$3\U$4

.this sentence
and this
This is already
.this sentence
and this.also this. and this.This one is not.