Regex allow a string to only contain numbers 0 - 9 and limit length to 45

Question

I am trying to create a regex to have a string only contain 0-9 as the characters and it must be at least 1 char in length and no more than 45. so

Answer 1

For this case word boundary (\b) can also be used instead of start anchor (^) and end anchor ($):

\b\d{1,45}\b

\b is a position between \w and \W (non-word char), or at the beginning or end of a string.