grep -E

https://man7.org/linux/man-pages/man1/grep.1.html

    -E, --extended-regexp
             Interpret PATTERNS as extended regular expressions (EREs,
             see below).

grep -E ' 50[0-9] '

See also

• ack, ag, grep, egrep, fgrep, agrep, ngrep, pgrep, awk, sed, strings, tr, tail, mtail, git grep, wc, uniq, LogQL, findstr (Windows), rg, git-grep, cut
• 5XX, 500, 502, 503, 504, 526, Web troubleshooting
• Regexp