Commentary
Charles Cawley says the primary source which confirms his parentage has not yet been identified, although his naming his son Giselbert suggests that this affiliation is probably correct.

Weis lists Gisebert and Ermengarge as Regnier I's parents. Stewart Baldwin lists several reasons this is not certain:

"1. There is an individual roughly one generation before Regnier who seems to be related, i.e., the same-named lay-abbot of Echternach from 864 to 870. Based on the available evidence, a scenario in which the two Regniers were father and son is difficult to rule out.
2. Duke Giselbert was married in 929, 83 years after his supposed grandfather carried off the daughter of Lothair I in 846. This seems to leave enough room for another generation, so that even if duke Giselbert was a descendant of the older Giselbert and Lothair's daughter, he might not have been their paternal grandson.
3. Nothing is known about the ancestry of Regnier's wife Alberada, the mother of duke Giselbert. In the absence of such knowledge, it is difficult to rule out the possibility that Giselbert was named after someone in his mother's family."
Family
ClaimDetailEvidence
Spouse 2ndAlberada (e860-916) 
Child +Giselbert, Duke of Lorraine (~885-939) [S756] [S918] [S2720]
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Child +Regnier II, Count of Hainaut (~885-932) [S756] [S2720]
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Attributes
ClaimDetailEvidence
GenderMale
NameRegnier I [S756:139]
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NameCount Reignier I Lay-abbot of Echternach [S1434]
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NameReginar [I] `Langhals Longneck` [S918] [S2720]
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NameRegnier Langhals I, Count of Hainaut
Nickname: Reginar Longneck
no source
TitleCount of Hainaut [S756] [S918] [S2720]
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Marriage 1st [S2720]
Hersent of France, dau. of Charles II, King of France
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Timeline
ClaimDateDetailAgeEvidence
Birthabt 850France [S756] [S918] [S2720]
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Flourished 877 - 886 [S756] [S918]
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LivingJun 14, 877 [S918]
mentioned in an agreement of Emporer Charles II
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Livingaft 886 [S918]
confirmed the donation of revenue by charter
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Title 897 to 915Lay Abbot of Echternach [S756] [S918]
Luxembourg
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Death Aug 25, 915 or Jan 19, 916Meerssen, France [S918] [S2720]
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Death Oct 25, 915 - Jan 19, 915/16 [S756]
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Sources
IndexTitle
[S756] Weis, Frederick Lewis, "Ancestral Roots of Sixty Colonists Who Came To New England before 1700, 7th Ed." (Genealogical Publishing Co., Inc., Baltimore, 1999).
[S918] Cawley, Charles, "Foundation of Medieval Genealogy (Medieval Lands - Lower Lotharingian Nobility)" (2006-7 (v1.1 Updated 21 December 2006)).
[S1434] Baldwin, Stewart, "The Henry Project - Regnier I" (April 24, 2008) (https://fasg.org/projects/henryproject/data/regin001.htm).
[S2720] Cawley, Charles, "Foundation of Medieval Genealogy (Medieval Lands - Hainaut)" (v4.6 Updated 03 February 2020) (https://fmg.ac/Projects/MedLands/HAINAUT.htm).