A single number in brackets

A single number in brackets, {n}, matches exactly n occurrence of the preceding expression. For example, the following regular expression matches the line consisting of exactly 5 digits between a and b:

$ awk '/a[0-9]{5}b/'  interval_regex.txt

The output on execution of the preceding code is as follows:

a12345b

If from the emp.dat file, you want to print all the lines in which an employee's phone number had the 9 repeated twice, we can write the regular expression as follows:

$ awk '/[9]{2}/' emp.dat

The output on execution of the preceding code is as follows:

Amit    Sharma  9911887766  amit@yahoo.com      M   lgs     2350
John    Kapur   9911556789  john@gmail.com      M   hr      2200
Billy   Chabra  9911664321  bily@yahoo.com      M   lgs     1900