Regular Expressions 101

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^[0-9]{20}
/
^ asserts position at start of the string
Match a single character present in the list below
[0-9]
{20} matches the previous token exactly 20 times
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String