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As @JeffSchaller says, awk is the tool for the job and since OP wants regex we can just combine the two

awk -F: '$4 ~ /^100[13]$/' mypasswd

and that allows a little golf putt on the grep version

grep -E "^(.*:){3}100[13]:" mypasswd

Edit

Anchored in accordance with @ikkachu comment

As @JeffSchaller says, awk is the tool for the job and since OP wants regex we can just combine the two

awk -F: '$4 ~ /^100[13]$/' mypasswd

and that allows a little golf putt on the grep version

grep -E "^(.*:){3}100[13]:" mypasswd

As @JeffSchaller says, awk is the tool for the job and since OP wants regex we can just combine the two

awk -F: '$4 ~ /100[13]/' mypasswd

and that allows a little golf putt on the grep version

grep -E "^(.*:){3}100[13]" mypasswd

As @JeffSchaller says, awk is the tool for the job and since OP wants regex we can just combine the two

awk -F: '$4 ~ /^100[13]$/' mypasswd

and that allows a little golf putt on the grep version

grep -E "^(.*:){3}100[13]:" mypasswd

Edit

Anchored in accordance with @ikkachu comment

As @JeffSchaller says, awk is the tool for the job and since OP wants regex we can just combine the two

awk -F: '$4 ~ /100[13]/' mypasswd

As @JeffSchaller says, awk is the tool for the job and since OP wants regex we can just combine the two

awk -F: '$4 ~ /100[13]/' mypasswd

and that allows a little golf putt on the grep version

grep -E "^(.*:){3}100[13]" mypasswd