How can a person be regenerated in the Old Testament if the spirit could leave him as he did Saul and Samson and almost David? (For if any man have not the spirit of Christ he is none of his)

As I am sure you know, regeneration is the doctrine that refers to being born again by the Spirit at the time of salvation. However, you may be confusing regeneration with the universal indwelling of the Spirit. Let me make some points.

1. There is no doubt that a change has been made in the indwelling of believers by the Spirit of God. This indwelling was only occasional in the Old Testament and only becomes universal (meaning it is the experience of all believers) in the New Testament (as per Romans 8:9But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
   See All...). Jesus clearly refers to this dispensational change in John 7:38-39 [38] He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water. [39] (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.)
   See All... and John 14:17Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
   See All....
2. However, there may also be a dispensational difference concerning regeneration. It is true that regeneration is difficult to establish in the Old Testament. But it is important to note that one could be born of the Spirit and yet not be indwelled by the Spirit. However, there may be prophetic references to the new birth in the passages on the New Covenant (see Jeremiah 31:33But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
   See All...; Ezekiel 11:19And I will give them one heart, and I will put a new spirit within you; and I will take the stony heart out of their flesh, and will give them an heart of flesh:
   See All...; 18:31
   See All...). Consider the following:

Ezekiel 36:26A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.
See All... A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.  27 And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.

These passages do not mention the new birth by name, but they seem to anticipate such a change of heart as caused by the new birth. In fact, when Nicodemus admitted that he knew nothing of the new birth, Jesus asked him, "Art thou a master of Israel, and knowest not these things?" Jesus assumed that a master (as in master teacher) in Israel should know such things. Again, I believe that this refers to the prophecies concerning the New Covenant.

But, does this mean that the Old Testament saints were regenerated? Not necessarily. I personally believe that it is a dispensation difference. Old Testament saints were probably not born of the Spirit as we are today. But here is where it gets interesting. They probably knew something of a salvation experience anyway. It was certainly not identical to our present experience: that is, trusting in the death, burial, and resurrection of Jesus Christ as the basis of our salvation. However, there was something. For instance, consider these verses:

1 Samuel 2:12Now the sons of Eli were sons of Belial; they knew not the LORD.
See All... Now the sons of Eli were sons of Belial; they knew not the LORD.

1 Samuel 3:7Now Samuel did not yet know the LORD, neither was the word of the LORD yet revealed unto him.
See All... Now Samuel did not yet know the LORD, neither was the word of the LORD yet revealed unto him.

In 1 Samuel 2:12Now the sons of Eli were sons of Belial; they knew not the LORD.
See All..., we see that the sons of Eli "knew not the LORD." I think we could definitely say that they were lost. In 1 Samuel 3:7Now Samuel did not yet know the LORD, neither was the word of the LORD yet revealed unto him.
See All..., we see that Samuel in his youth had a time when he "did not yet know the LORD." Knowing the life and testimony of Samuel, I am sure that he knew the Lord at a later time. Therefore, there must have been a time when he came to know the Lord. This may not be regeneration, but it certainly sounds like some form of salvation.

In Hebrews 11:13These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.
See All..., in the midst of the chapter dealing with Old Testament saints, we are told that these "all died in faith." This clearly means something distinct. Were they born in faith? I think not. That would not make sense. But if they were not born in faith but died in faith, then there must have been a time when they came to be "in faith." Again, this would compare to our concept of salvation.

I do not believe that all the doctrines connected with salvation today have always been connected with salvation. But I do believe that some form of salvation was known even to the saints of Old Testament times. I am afraid that some people see some differences and then assume that nothing can cross dispensational lines. With this approach I disagree.