(?(DEFINE)
    (?<expr>
        (?&number)
        (?:
            \s*
            (?:
                (?&number)
            |   (?&unary_op)
            |   (?&binary_op)
            )
        )*
    )
    (?<number>
        (?> \d+ (?: [.] \d* )? | [.] \d+ )
        (?{ $^R + 1})
    )
    (?<unary_op>
        (?i: chs | abs | sqr | sqrt | sin | cos | tan ) 
        (?![a-zA-Z])
    )
    (?<binary_op>
        [-+/*^]
        (?{ $^R - 1 })
    )
)

    \A \s* (?&expr) \s* \z
    (?(?{ $^R != 1 }) (*FAIL) ) 
}msx