Regular Expressions 101

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/
^\d{11}$
/
g
^ asserts position at start of the string
\d
matches a digit (equivalent to [0-9])
{11} matches the previous token exactly 11 times
$ asserts position at the end of the string
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
g

Test String