Regular Expressions 101

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^5[1-5][0-9]{14}|5[0-9][0-9]{14}$
/
1st Alternative
^5[1-5][0-9]{14}
^ asserts position at start of the string
5 matches the character 5 with index 5310 (3516 or 658) literally (case sensitive)
Match a single character present in the list below
[1-5]
1-5 matches a single character in the range between 1 (index 49) and 5 (index 53) (case sensitive)
Match a single character present in the list below
[0-9]
{14} matches the previous token exactly 14 times
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
2nd Alternative
5[0-9][0-9]{14}$
5 matches the character 5 with index 5310 (3516 or 658) literally (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
{14} matches the previous token exactly 14 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
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/

Test String