text: user6511: The following is a valid .fasta file content:
  >HSBGPG Human gene for bone gla protein (BGP) GGCAGATTCCCCCTAGACCCGCCCGCACCATGGTCAGGCATGCCCCTCCTCATCGCTGGGCACAGCCCAGAGGGT ATAAACAGTGCTGGAGGCTGGCGGGGCAGGCCAGCTGAGTCCTGAGCAGCAGCCCAGCGCAGCCACCGAGACACC ATGAG.
  Is this also?
  >Arbirary_Name_iJustCameUP_with_and_other_local_identifiers GGCAGATTCCCCCTAGACCCGCCCGCACCATGGTCAGGCATGCCCCTCCTCATCGCTGGGCACAGCCCAGAGGGT ATAAACAGTGCTGGAGGCTGGCGGGGCAGGCCAGCTGAGTCCTGAGCAGCAGCCCAGCGCAGCCACCGAGACACC ATGAG.
  
  user9384: NCBI's FASTA format description:
  https://www.ncbi.nlm.nih.gov/genbank/fastaformat/
  NCBI's BLAST page describing valid FASTA input:
  https://blast.ncbi.nlm.nih.gov/Blast.cgi?CMD=Web&PAGE_TYPE=BlastDocs&DOC_TYPE=BlastHelp
  NCBI's SNP page describing FASTA format:
  https://www.ncbi.nlm.nih.gov/projects/SNP/snp_legend.cgi?legend=fasta
  UniProt's FASTA-header description:
  https://www.uniprot.org/help/fasta-headers
  Wikipedia reference (indicating format origin):
  https://en.wikipedia.org/wiki/FASTA_format
  FASTA program (origin of format, per Wikipedia):
  https://fasta.bioch.virginia.edu/wrp_fasta/fasta_guide.pdf
  Harvard PolyPhen page describing FASTA format:
  http://genetics.bwh.harvard.edu/pph/FASTA.html
  FASTA file format (from Pacific Biosciences):
  https://pacbiofileformats.readthedocs.io/en/3.0/FASTA.html
  
  user3541: I am not aware of an official fasta format description. The only constraint that I know of is that there should not be any whitespace after >. Apart from that particular position, whitespaces are allowed in the header line.
  So, regarding your particular example, both are valid fasta sequences. When in doubt, you can use SeqIO from Biopython, if you can parse your file with the following code, it  is should be a valid fasta file.
  from Bio import SeqIO
  
  with open("example.fasta") as handle:
      for record in SeqIO.parse(handle, "fasta"):
          print(record.id)
  
  Edit per @Chris_Rands' comment
  The code below does the same as above, meaning that SeqIO.parse() takes care of opening and closing the file.
  from Bio import SeqIO
  
  for record in SeqIO.parse("example.fasta", "fasta"):
      print(record.id)
  
  
original_text: 6511: <p>The following is a valid .fasta file content:</p>
  <p><code>&gt;HSBGPG Human gene for bone gla protein (BGP) GGCAGATTCCCCCTAGACCCGCCCGCACCATGGTCAGGCATGCCCCTCCTCATCGCTGGGCACAGCCCAGAGGGT ATAAACAGTGCTGGAGGCTGGCGGGGCAGGCCAGCTGAGTCCTGAGCAGCAGCCCAGCGCAGCCACCGAGACACC ATGAG.</code></p>
  <p>Is this also?</p>
  <p><code>&gt;Arbirary_Name_iJustCameUP_with_and_other_local_identifiers GGCAGATTCCCCCTAGACCCGCCCGCACCATGGTCAGGCATGCCCCTCCTCATCGCTGGGCACAGCCCAGAGGGT ATAAACAGTGCTGGAGGCTGGCGGGGCAGGCCAGCTGAGTCCTGAGCAGCAGCCCAGCGCAGCCACCGAGACACC ATGAG.</code></p>
   9384: <p>NCBI's FASTA format description:</p>
  <p><a href="https://www.ncbi.nlm.nih.gov/genbank/fastaformat/" rel="nofollow noreferrer">https://www.ncbi.nlm.nih.gov/genbank/fastaformat/</a></p>
  <p>NCBI's BLAST page describing valid FASTA input:</p>
  <p><a href="https://blast.ncbi.nlm.nih.gov/Blast.cgi?CMD=Web&amp;PAGE_TYPE=BlastDocs&amp;DOC_TYPE=BlastHelp" rel="nofollow noreferrer">https://blast.ncbi.nlm.nih.gov/Blast.cgi?CMD=Web&amp;PAGE_TYPE=BlastDocs&amp;DOC_TYPE=BlastHelp</a></p>
  <p>NCBI's SNP page describing FASTA format:</p>
  <p><a href="https://www.ncbi.nlm.nih.gov/projects/SNP/snp_legend.cgi?legend=fasta" rel="nofollow noreferrer">https://www.ncbi.nlm.nih.gov/projects/SNP/snp_legend.cgi?legend=fasta</a></p>
  <p>UniProt's FASTA-header description:</p>
  <p><a href="https://www.uniprot.org/help/fasta-headers" rel="nofollow noreferrer">https://www.uniprot.org/help/fasta-headers</a></p>
  <p>Wikipedia reference (indicating format origin):</p>
  <p><a href="https://en.wikipedia.org/wiki/FASTA_format" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/FASTA_format</a></p>
  <p>FASTA program (origin of format, per Wikipedia):</p>
  <p><a href="https://fasta.bioch.virginia.edu/wrp_fasta/fasta_guide.pdf" rel="nofollow noreferrer">https://fasta.bioch.virginia.edu/wrp_fasta/fasta_guide.pdf</a></p>
  <p>Harvard <em>PolyPhen</em> page describing FASTA format:</p>
  <p><a href="http://genetics.bwh.harvard.edu/pph/FASTA.html" rel="nofollow noreferrer">http://genetics.bwh.harvard.edu/pph/FASTA.html</a></p>
  <p>FASTA file format (from Pacific Biosciences):</p>
  <p><a href="https://pacbiofileformats.readthedocs.io/en/3.0/FASTA.html" rel="nofollow noreferrer">https://pacbiofileformats.readthedocs.io/en/3.0/FASTA.html</a></p>
   3541: <p>I am not aware of an official fasta format description. <strike>The only constraint that I know of is that there should not be any whitespace after <code>&gt;</code></strike>. Apart from that particular position, whitespaces are allowed in the header line.</p>
  <p>So, regarding your particular example, both are valid fasta sequences. When in doubt, you can use <a href="https://biopython.org/wiki/SeqIO" rel="nofollow noreferrer"><code>SeqIO</code></a> from <code>Biopython</code>, if you can parse your file with the following code, it <strike> is</strike> should be a valid fasta file.</p>
  <pre><code>from Bio import SeqIO
  
  with open(&quot;example.fasta&quot;) as handle:
      for record in SeqIO.parse(handle, &quot;fasta&quot;):
          print(record.id)
  </code></pre>
  <p>Edit per @Chris_Rands' comment</p>
  <p>The code below does the same as above, meaning that <code>SeqIO.parse()</code> takes care of opening and closing the file.</p>
  <pre><code>from Bio import SeqIO
  
  for record in SeqIO.parse(&quot;example.fasta&quot;, &quot;fasta&quot;):
      print(record.id)
  </code></pre>
  