Add validUndefinedVariableRegexp to README (#192)

Author: sirbrillig

README.md

@@ -69,7 +69,8 @@ The available options are as follows:
 - `allowUnusedParametersBeforeUsed` (bool, default `true`): if set to true, unused function arguments will be ignored if they are followed by used function arguments.
 - `validUnusedVariableNames` (string, default `null`): a space-separated list of names of placeholder variables that you want to ignore from unused variable warnings. For example, to ignore the variables `$junk` and `$unused`, this could be set to `'junk unused'`.
 - `ignoreUnusedRegexp` (string, default `null`): a PHP regexp string (note that this requires explicit delimiters) for variables that you want to ignore from unused variable warnings. For example, to ignore the variables `$_junk` and `$_unused`, this could be set to `'/^_/'`.
-- `validUndefinedVariableNames` (string, default `null`): a space-separated list of names of placeholder variables that you want to ignore from undefined variable warnings. For example, to ignore the variables `$post` and `$undefined`, this could be set to `'post undefined'`.
+- `validUndefinedVariableNames` (string, default `null`): a space-separated list of names of placeholder variables that you want to ignore from undefined variable warnings. For example, to ignore the variables `$post` and `$undefined`, this could be set to `'post undefined'`. This can be used in combination with `validUndefinedVariableRegexp`.
+- `validUndefinedVariableRegexp` (string, default `null`): a PHP regexp string (note that this requires explicit delimiters) for variables that you want to ignore from undefined variable warnings. For example, to ignore the variables `$post` and `$undefined`, this could be set to `'/^(post|undefined)$/'`. This can be used in combination with `validUndefinedVariableNames`.
 - `allowUnusedForeachVariables` (bool, default `true`): if set to true, unused values from the `key => value` syntax in a `foreach` loop will never be marked as unused.
 - `sitePassByRefFunctions` (string, default `null`): a list of custom functions which pass in variables to be initialized by reference (eg `preg_match()`) and therefore should not require those variables to be defined ahead of time. The list is space separated and each entry is of the form `functionName:1,2`. The function name comes first followed by a colon and a comma-separated list of argument numbers (starting from 1) which should be considered variable definitions. The special value `...` in the arguments list will cause all arguments after the last number to be considered variable definitions.
 - `allowWordPressPassByRefFunctions` (bool, default `false`): if set to true, a list of common WordPress pass-by-reference functions will be added to the list of PHP ones so that passing undefined variables to these functions (to be initialized by reference) will be allowed.