Regular Expressions 101

\ba Unicode word boundary

Character Class[1-9]

1-9one character from 1 (49₁₀) to 9 (57₁₀)

\d{0,2}a Unicode decimal digit, repeated between 0 and 2 times

Non-Capturing Group(?:[ ,.]\d{3})*

*repeats the group contents below any number of times:

Character Class[ ,.]

the literal (32₁₀ / 40₈ / 20₁₆)

,the literal , (44₁₀ / 54₈ / 2C₁₆)

.the literal . (46₁₀ / 56₈ / 2E₁₆)

\d{3}a Unicode decimal digit, repeated exactly 3 times

\ba Unicode word boundary

Global pattern flags

u modifier: unicode. Treats the pattern and subject as UTF-16

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

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

\b[1-9]\d{0,2}(?:[ ,.]\d{3})*\b

New

it need to only select , when there is number around it. for example only 
when 123,456 i need to select and replace "," I'm converting English
numbers into Persian (e.g: "Hello 123" becomes "Hello ۱۲۳"). now I need to
replace the Decimal separator with Persian version too. but I don't know how
I can select it with regex. e.g: "Hello 121,534" most become 
"Hello ۱۲۱/۵۳۴" The character that needs to be replaced is , with /