How explanations work

Regex guidance compiles JavaScript syntax and walks common tokens without executing the pattern. It describes groups, character classes, escapes, quantifiers and assertions. Cron uses the site’s five-field parser and describes each field, including the day-of-month/weekday OR rule. SQL combines formatter diagnostics with common database-error explanations and a clause guide; it never executes the SQL.

Know the limits

Parser explanations are guidance, not engine equivalence or a security proof. Advanced Unicode regex set syntax, dialect-specific SQL and schema-dependent errors require verification in the actual runtime. Local parser limits are 20,000 regex characters, 50,000 SQL characters and 10,000 error characters.

Use Regex Tester for cancellable matching, Cron Expression Generator for upcoming runs, and SQLite Database Lab to execute a query against a local schema.

Example: read a regular expression

Select Regex, enter ^\d{2,4}$ and use the u flag. The table uses the same parser as the tool. It describes a two-to-four digit pattern; it does not run it against text.

Token	Explanation
`^`	Assert the beginning of the input (or line with m).
`\d`	Match digit.
`{2,4}`	Repeat the preceding atom 2 to 4 times, using greedy matching.
`$`	Assert the end of the input (or line with m).

Example: weekday Cron schedule

0 9 * * 1-5

At 09:00, Monday through Friday. The hour belongs to the scheduler’s configured time zone, not necessarily UTC. Open the weekday schedule guide only when its expression matches your intended schedule, or enter this expression in Cron Expression Generator to inspect upcoming runs.

Example: a missing SQL table

SELECT name FROM users;

In SQL & errors mode, supply no such table: users as the database error. The diagnostic is:

The referenced table is missing from the connected database or schema. Check spelling, database selection, schema qualification and whether it has been imported.

No database is connected by the explanation itself. If the missing source is a local CSV or Parquet file, DuckDB Playground can import it and display its actual table name before you run a query. Database-specific permissions, schemas and unsupported syntax still need checking in the target engine.

Documentation examples checked: 2026-10-24. How these tools are checked.

Sources

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