1. A computer-implemented method performed by at least one processor, the method comprising: identifying an expression for validation by a parser, the expression represented as a natural language input including a set of phrases; iteratively converting each of the phrases of the natural language input into a set of tokens, wherein converting each phrase includes: determining whether a current phrase matches a pre-defined token of a plurality of pre-defined tokens associated with a known grammar rule; in response to determining that the current phrase matches a pre-defined token associated with a known grammar rule, generating a token for the current phrase for use in validating the expression; in response to determining that the current phrase does not match a pre-defined token associated with a known grammar rule, the current phrase comprising an unrecognized string of characters: comparing the unrecognized string of characters to a plurality of dynamic tokens included in an external pool of tokens, the plurality of dynamic tokens included in the external pool of tokens different than the plurality of pre-defined tokens associated with the known grammar rule; identifying a dynamic token from the external pool of tokens corresponding to the unrecognized string of characters; identifying a type of the identified dynamic token, wherein the identified type is one of a string, a number, or a date; and generating a generic token for the current phrase based on the identified type of the identified dynamic token, wherein the generic token is a character string corresponding to the identified type of the identified dynamic token; and replacing the identified dynamic token with the generic token for use in validating the expression; generating a tokenized expression by combining the generated tokens; and validating the tokenized expression.