Here is the Merchant Center Query Language grammar reference (in regular expression notation):

```
Query            -> SelectClause FromClause? WhereClause? OrderByClause? LimitClause?
SelectClause     -> SELECT FieldName (, FieldName)*
FromClause       -> FROM TableName
WhereClause      -> WHERE Condition (AND Condition)*
OrderByClause    -> ORDER BY Ordering (, Ordering)*
LimitClause      -> LIMIT PositiveInteger

Condition        -> FieldName Operator Value | FieldName BETWEEN Value AND Value
Operator         -> = | != | > | >= | < | <= | <> | IN | NOT IN |
                    CONTAINS ANY | CONTAINS ALL | CONTAINS NONE | DURING |
                    LIKE | NOT LIKE | REGEXP_MATCH | NOT REGEXP_MATCH
Value            -> Number | NumberList | String | StringList | Function
Ordering         -> FieldName (ASC | DESC)?

FieldName        -> [a-z] ([a-zA-Z0-9._])*
TableName        -> [A-Z] ([a-zA-Z_])*

StringList       -> ( String (, String)* )
NumberList       -> ( Number (, Number)* )

PositiveInteger  -> [1-9] ([0-9])*
Number           -> -? [0-9]+ (. [0-9] [0-9]*)?
String           -> (' Char* ') | (" Char* ")
Function         -> LAST_14_DAYS | LAST_30_DAYS | LAST_7_DAYS |
                    LAST_BUSINESS_WEEK | LAST_MONTH | LAST_WEEK_MON_SUN |
                    LAST_WEEK_SUN_SAT | THIS_MONTH | THIS_WEEK_MON_TODAY |
                    THIS_WEEK_SUN_TODAY | TODAY | YESTERDAY
```

- `?`indicates an optional element.
- `*` means zero or more; `+` means one or more.
- `(xxxxxx)` indicates a grouping.
- `[a-z0-9]` signifies character ranges.
- `|` stands for "or".

> [!IMPORTANT]
> **Key Point:** The `REGEXP_MATCH` operator uses [RE2 syntax](https://github.com/google/re2/wiki/Syntax).