grep1-3  Exercise 1: Date, Time, Then ERROR or WARN
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cat > server.log <<'EOF'
2026-10-08 09:14:02 INFO  started
2026-10-08 09:15:11 WARN  slow query
2026-10-08 09:15:48 ERROR disk full
10-08 09:20:00 ERROR bad date format
2026-10-08 09:21:30 error lowercase
  2026-10-08 09:22:00 ERROR indented
2026-10-08 09:30:00 WARNING not a WARN
2026-10-08 09:31:00 ERROR timeout
EOF

grep -E '^[0-9]{4}-[0-9]{2}-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2} (ERROR|WARN) ' server.log

Output:

2026-10-08 09:15:11 WARN  slow query
2026-10-08 09:15:48 ERROR disk full
2026-10-08 09:31:00 ERROR timeout

WHY THIS WORKS AS AN ANSWER
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^ pins the start, so the indented line (which begins with spaces) is out. {4}, {2} and {2} after [0-9] insist on a full date and time, so '10-08 09:20:00' is out. (ERROR|WARN) is a group with an either-or, which needs -E so that the | and the parentheses are operators without backslashes. The final space is what excludes WARNING: the pattern needs WARN followed by a space, and WARNING has an I there. Lowercase 'error' is out because the match is case-sensitive.
Three lines remain: the WARN and the two ERRORs with a clean timestamp. -w would NOT do this job: it would accept the indented line, and the date format is not checked.