[DOCS] EQL: Add case-insensitive `~` operator (#68217)

Documents the case-insensitive `~` operator for `in` and string functions.

Relates to #67869 and #68176

docs/reference/eql/functions.asciidoc

@@ -6,8 +6,7 @@
 <titleabbrev>Function reference</titleabbrev>
 ++++
 
-{es} supports the following <<eql-functions,EQL functions>>. Most EQL functions
-are case-sensitive by default.
+{es} supports the following <<eql-functions,EQL functions>>.
 
 [discrete]
 [[eql-fn-add]]
@@ -61,15 +60,19 @@ If using a field as the argument, this parameter supports only
 === `between`
 
 Extracts a substring that's between a provided `left` and `right` text in a
-source string. Matching is case-sensitive.
+source string. Matching is case-sensitive by default.
 
 *Example*
 [source,eql]
 ----
 // file.path = "C:\\Windows\\System32\\cmd.exe"
-between(file.path, "system32\\\\", ".exe")                // returns "cmd"
+between(file.path, "System32\\\\", ".exe")                // returns "cmd"
+between(file.path, "system32\\\\", ".exe")                // returns ""
 between(file.path, "workspace\\\\", ".exe")               // returns ""
 
+// Make matching case-insensitive
+between~(file.path, "system32\\\\", ".exe")               // returns "cmd"
+
 // Greedy matching defaults to false.
 between(file.path, "\\\\", "\\\\", false)                 // returns "Windows"
 
@@ -77,11 +80,11 @@ between(file.path, "\\\\", "\\\\", false)                 // returns "Windows"
 between(file.path, "\\\\", "\\\\", true)                  // returns "Windows\\System32"
 
 // empty source string
-between("", "system32\\\\", ".exe")                       // returns ""
+between("", "System32\\\\", ".exe")                       // returns ""
 between("", "", "")                                       // returns ""
 
 // null handling
-between(null, "system32\\\\", ".exe")                     // returns null
+between(null, "System32\\\\", ".exe")                     // returns null
 ----
 
 *Syntax*
@@ -340,15 +343,19 @@ If using a field as the argument, this parameter supports only
 === `endsWith`
 
 Returns `true` if a source string ends with a provided substring. Matching is
-case-sensitive.
+case-sensitive by default.
 
 *Example*
 [source,eql]
 ----
 endsWith("regsvr32.exe", ".exe")          // returns true
+endsWith("regsvr32.exe", ".EXE")          // returns false
 endsWith("regsvr32.exe", ".dll")          // returns false
 endsWith("", "")                          // returns true
 
+// Make matching case-insensitive
+endsWith~("regsvr32.exe", ".EXE")         // returns true
+
 // file.name = "regsvr32.exe"
 endsWith(file.name, ".exe")               // returns true
 endsWith(file.name, ".dll")               // returns false
@@ -405,7 +412,7 @@ field data types:
 === `indexOf`
 
 Returns the first position of a provided substring in a source string. Matching
-is case-sensitive.
+is case-sensitive by default.
 
 If an optional start position is provided, this function returns the first
 occurrence of the substring at or after the start position.
@@ -414,11 +421,16 @@ occurrence of the substring at or after the start position.
 [source,eql]
 ----
 // url.domain = "subdomain.example.com"
+indexOf(url.domain, "d")        // returns 3
+indexOf(url.domain, "D")        // returns null
 indexOf(url.domain, ".")        // returns 9
 indexOf(url.domain, ".", 9)     // returns 9
 indexOf(url.domain, ".", 10)    // returns 17
 indexOf(url.domain, ".", -6)    // returns 9
 
+// Make matching case-insensitive
+indexOf~(url.domain, "D")        // returns 4
+
 // empty strings
 indexOf("", "")                 // returns 0
 indexOf(url.domain, "")         // returns 0
@@ -748,15 +760,19 @@ Fields are not supported as arguments.
 === `startsWith`
 
 Returns `true` if a source string begins with a provided substring. Matching is
-case-sensitive.
+case-sensitive by default.
 
 *Example*
 [source,eql]
 ----
 startsWith("regsvr32.exe", "regsvr32")  // returns true
+startsWith("regsvr32.exe", "Regsvr32")  // returns false
 startsWith("regsvr32.exe", "explorer")  // returns false
 startsWith("", "")                      // returns true
 
+// Make matching case-insensitive
+startsWith~("regsvr32.exe", "Regsvr32")  // returns true
+
 // process.name = "regsvr32.exe"
 startsWith(process.name, "regsvr32")    // returns true
 startsWith(process.name, "explorer")    // returns false
@@ -848,16 +864,20 @@ If using a field as the argument, this parameter does not support the
 === `stringContains`
 
 Returns `true` if a source string contains a provided substring. Matching is
-case-sensitive.
+case-sensitive by default.
 
 *Example*
 [source,eql]
 ----
 // process.command_line = "start regsvr32.exe"
 stringContains(process.command_line, "regsvr32")  // returns true
+stringContains(process.command_line, "Regsvr32")  // returns false
 stringContains(process.command_line, "start ")    // returns true
 stringContains(process.command_line, "explorer")  // returns false
 
+// Make matching case-insensitive
+stringContains~(process.command_line, "Regsvr32")  // returns false
+
 // process.name = "regsvr32.exe"
 stringContains(command_line, process.name)        // returns true
 
@@ -1008,7 +1028,7 @@ If using a field as the argument, this parameter supports only
 === `wildcard`
 
 Returns `true` if a source string matches one or more provided wildcard
-expressions. Matching is case-sensitive.
+expressions. Matching is case-sensitive by default.
 
 *Example*
 [source,eql]
@@ -1016,10 +1036,14 @@ expressions. Matching is case-sensitive.
 // The * wildcard matches zero or more characters.
 // process.name = "regsvr32.exe"
 wildcard(process.name, "*regsvr32*")                // returns true
+wildcard(process.name, "*Regsvr32*")                // returns false
 wildcard(process.name, "*regsvr32*", "*explorer*")  // returns true
 wildcard(process.name, "*explorer*")                // returns false
 wildcard(process.name, "*explorer*", "*scrobj*")    // returns false
 
+// Make matching case-insensitive
+wildcard~(process.name, "*Regsvr32*")                // returns true
+
 // The ? wildcard matches exactly one character.
 // process.name = "regsvr32.exe"
 wildcard(process.name, "regsvr32.e?e")                  // returns true

docs/reference/eql/syntax.asciidoc

@@ -220,7 +220,9 @@ Returns `true` if the condition to the right is `false`.
 [source,eql]
 ----
 user.name in ("Administrator", "SYSTEM", "NETWORK SERVICE")
+user.name in~ ("administrator", "system", "network service")
 user.name not in ("Administrator", "SYSTEM", "NETWORK SERVICE")
+user.name not in~ ("administrator", "system", "network service")
 user.name : ("administrator", "system", "network service")
 ----
 
@@ -228,10 +230,18 @@ user.name : ("administrator", "system", "network service")
 Returns `true` if the value is contained in the provided list. For strings,
 matching is case-sensitive.
 
+`in~` (case-insensitive)::
+Returns `true` if the value is contained in the provided list. For strings,
+matching is case-insensitive.
+
 `not in` (case-sensitive)::
 Returns `true` if the value is not contained in the provided list. For strings,
 matching is case-sensitive.
 
+`not in~` (case-insensitive)::
+Returns `true` if the value is not contained in the provided list. For strings,
+matching is case-insensitive.
+
 `:` (case-insensitive)::
 Returns `true` if the value is contained in the provided list. Can only be used
 to compare strings.
@@ -627,12 +637,25 @@ until [ process where event.type == "stop" ]
 === Functions
 
 You can use EQL functions to convert data types, perform math, manipulate
-strings, and more. Most functions are case-sensitive by default.
+strings, and more. For a list of supported functions, see <<eql-function-ref>>.
 
-For a list of supported functions, see <<eql-function-ref>>.
+[discrete]
+[[eql-case-insensitive-functions]]
+=== Case-insensitive functions
+
+Most EQL functions are case-sensitive by default. To make a function
+case-insensitive, use the `~` operator after the function name:
+
+[source,eql]
+----
+stringContains(process.name,".exe")  // Matches ".exe" but not ".EXE" or ".Exe"
+stringContains~(process.name,".exe") // Matches ".exe", ".EXE", or ".Exe"
+----
+
+[discrete]
+[[eql-how-functions-impact-search-performance]]
+=== How functions impact search performance
 
-[TIP]
-====
 Using functions in EQL queries can result in slower search speeds. If you
 often use functions to transform indexed data, you can speed up search by making
 these changes during indexing instead. However, that often means slower index
@@ -672,7 +695,6 @@ file where file.extension in ("exe", "dll")
 
 We recommend testing and benchmarking any indexing changes before deploying them
 in production. See <<tune-for-indexing-speed>> and <<tune-for-search-speed>>.
-====
 
 [discrete]
 [[eql-pipes]]
@@ -741,8 +763,10 @@ sub-fields of a `nested` field. However, data streams and indices containing
 {es} EQL differs from the {eql-ref}/index.html[Elastic Endgame EQL syntax] as
 follows:
 
-* Most operators and functions in {es} EQL are case-sensitive. For
-case-insensitive equality comparisons, use the `:` operator.
+* Most operators and functions in {es} EQL are case-sensitive by default. For
+case-insensitive equality comparisons, use the `:` operator. To make a function
+case-insensitive, use the `~` operator after the function name. See
+<<eql-case-insensitive-functions>>.
 
 * Comparisons using the `==` and `!=` operators do not expand wildcard
 characters. For example, `process_name == "cmd*.exe"` interprets `*` as a