A search operator specifies how the comparison of a search value and a zettel should be executed. Every comparison is done case-insensitive, treating all uppercase letters the same as lowercase letters. The following are allowed search operator characters: * The exclamation mark character (“`!`”, U+0021) negates the meaning. * The equal sign character (“`=`”, U+003D) compares on equal content (“equals operator”). * The tilde character (“`~`”, U+007E) compares on matching (“match operator”). * The left square bracket character (“`[`”, U+005B) matches if there is some prefix (“prefix operator”). * The right square bracket character (“`]`”, U+005D) compares a suffix relationship (“suffix operator”). * The colon character (“`:`”, U+003A) compares depending on the actual [key type](00001006030000) (“has operator”). In most cases, it acts as an equals operator, but for some type it acts as the match operator. * The less-than sign character (“`<`”, U+003C) matches if the search value is somehow less than the metadata value (“less operator”). * The greater-than sign character (“`>`”, U+003E) matches if the search value is somehow greater than the metadata value (“greater operator”). * The question mark (“`?`”, U+003F) checks for an existing metadata key (“exist operator”). In this case no [search value](00001007706000) must be given. Since the exclamation mark character can be combined with the other operators, there are 18 possible combinations: 1. “`!`”: is an abbreviation of the “`!~`” operator. 1. “`~`”: is successful if the search value matches the value to be compared. 1. “`!~`”: is successful if the search value does not match the value to be compared. 1. “`=`”: is successful if the search value is equal to one word of the value to be compared. 1. “`!=`”: is successful if the search value is not equal to any word of the value to be compared. 1. “`[`”: is successful if the search value is a prefix of the value to be compared. 1. “`![`”: is successful if the search value is not a prefix of the value to be compared. 1. “`]`”: is successful if the search value is a suffix of the value to be compared. 1. “`!]`”: is successful if the search value is not a suffix of the value to be compared. 1. “`:`”: is successful if the search value is has/match one word of the value to be compared. 1. “`!:`”: is successful if the search value is not match/has to any word of the value to be compared. 1. “`<`”: is successful if the search value is less than the value to be compared. 1. “`!<`”: is successful if the search value is not less than, e.g. greater or equal than the value to be compared. 1. “`>`”: is successful if the search value is greater than the value to be compared. 1. “`!>`”: is successful if the search value is not greater than, e.g. less or equal than the value to be compared. 1. “`?`”: is successful if the metadata contains the given key. 1. “`!?`”: is successful if the metadata does not contain the given key. 1. “``”: a missing search operator can only occur for a full-text search. It is equal to the “`~`” operator.