Regular Expressions 101

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^[1-9]{1}[0-9]{3}$
/
^ asserts position at start of the string
Match a single character present in the list below
[1-9]
{1} matches the previous token exactly one time (meaningless quantifier)
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
{3} matches the previous token exactly 3 times
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String