The Filter That Crashed the Nightly Job

The Filter That Crashed the Nightly Job

Your extension's setup table has a text field where consultants type a customer filter — 1000..2000|3000, the platform's own filter idiom — and a nightly job later slams that text straight into SetFilter. Last Tuesday somebody saved 1000..2000| with a trailing pipe, and the job died at 02:13 with an error nobody saw until morning. You are writing the syntax check the setup page will run before accepting the value, so a malformed filter is rejected at entry time instead of at 2 AM.

Requirements

Create a codeunit named "Filter Expression Check" with one public procedure:

procedure IsValid(Expression: Text): Boolean

The procedure returns true when the expression follows the mini filter grammar below, and false otherwise — it must never raise an error, no matter how mangled the input is.

The grammar is a deliberate subset of Business Central's real filter syntax (<>, *, @, & and friends are out of scope — and therefore illegal here). All seven rules are graded:

1. An expression is one or more alternatives separated by |. Every alternative must be non-empty: a leading |, a trailing |, or a doubled || makes the expression invalid, and the empty expression is invalid too.
2. An alternative is either a group — a complete expression wrapped in parentheses, like (1000..2000|3000) — or a single value or range. A group is the whole alternative: it cannot be glued to other values or groups.
3. A range is low..high, low.., or ..high; every bound that is present must be a single value. A bare .., a second .. in the same alternative (1..2..3), or a group used as a bound ((1|2)..5) is invalid.
4. A value is either an unquoted token or a quoted value. An unquoted token is one or more letters A–Z/a–z or digits 0–9 — nothing else, no exceptions.
5. A quoted value is wrapped in single quotes '. Inside the quotes every character is plain data — |, parentheses, dots and spaces do not act as operators there. A literal quote inside is written doubled ('O''Brien'), and '' is a valid, empty value. A quote that never closes makes the expression invalid, and after a closing quote only .., |, ), or the end of the expression may follow.
6. Parentheses outside quotes must be balanced and properly nested, and a group must contain a valid expression — (), (1000..2000, 1000) and )( are all invalid.
7. Outside quotes, only these characters are legal: letters, digits, |, (, ), ', and . as one half of a .. operator. A space, a -, a lone dot (1.5) — anything else anywhere outside quotes — invalidates the whole expression.

The check is purely syntactic: whether low actually sorts before high, or whether any customer matches, is not its business.

What the tests check

The tests call IsValid on valid expressions — a single token, token alternatives, 1000..2000|3000, open-ended ranges in both directions, a quoted value full of operator characters, a doubled quote, the empty quoted value '', quoted range bounds 'A'..'M', groups, nested groups, and a parenthesis inside quotes that must not count toward balancing — and on invalid ones: the empty expression, leading/trailing/doubled |, bare .., 1..2..3, a space outside quotes, a hyphen, a wildcard *, an @, an &, a lone dot, an unclosed (, a stray ), )(, (), an unterminated quote, a quote "closed" only by the escape '', text glued after a closing quote, and a group used as a range bound. Two more expressions are assembled from randomized tokens — one valid, one corrupted — so hardcoding the examples fails.

Learn More

• Entering criteria in filters
• Record.SetFilter(Any, Text [, Any,...]) Method
• Filtering records with the SetRange, SetFilter, GetRangeMin, and GetRangeMax methods
• Text data type
• Text.CopyStr(Text, Integer [, Integer]) Method