Escaping Expression Language

There may be times when a property supports Expression Language, but the user wishes to use a literal value that follows the same syntax as the Expression Language. For example, a user may want to configure a property value to be the literal text Hello ${UserName}. In such a case, this can be accomplished by using an extra $ (dollar sign symbol) just before the expression to escape it (i.e., Hello $${UserName}). Unless the $ character is being used to escape an Expression, it should not be escaped. For example, the value Hello $$User$$Name should not escape the $ characters, so the literal value that will be used is Hello $$User$$Name.

If more than two $ characters are encountered sequentially before a {, then each pair of $ characters will be considered an escaping of the $ character. The escaping will be performed from left-to-right. To help illustrate this, consider that the variable abc contains the value xyz. Then, consider the following table of Expressions and their corresponding evaluated values:

Table 1. Escaping EL Examples
Expression	Value	Notes
`${abc}`	`xyz`
`$${abc}`	`${abc}`
`$$${abc}`	`$xyz`
`$$$${abc}`	`$${abc}`
`$$$$${abc}`	`$$xyz`
`I owe you $5`	`I owe you $5`	No actual Expression is present here.
`You owe me $$5 too`	`You owe me $$5 too`	The $ character is not escaped because it does not immediately precede an Expression.
`Unescaped $$${5 because no closing brace`	`Unescaped $$${5 because no closing brace`	Because there is no closing brace here, there is no actual Expression and hence the $ characters are not escaped.
`Unescaped $$${5} because no closing brace`	<Error>	This expression is not valid because it equates to an escaped $, followed by `${5}` and the `${5}` is not a valid Expression. The number must be escaped.
`Unescaped $$${'5'} because no closing brace`	`Unescaped $ because no closing brace`	There is no attribute named `5` so the Expression evaluates to an empty string. The `$$` evaluates to a single (escaped) `$` because it immediately precedes an Expression.