Python Regexes: Named Groups. Cool Bananas

I'm currently writing a Python parser for GenBank files. I know BioPython has one, and it doesn't even suck, but BioPython requires a bunch of C extensions, so I can't go and just ship it with my Python application. So I'm creating a BioPython-compatible API for the classes that I need for antiSMASH, without the dependency tail BioPython forces on me. Having contributed to BioPerl before, I do like to use regular expressions for token-based parsers, especially as I'm not too fond of lexers in Python. Now, a GenBank header is a peculiar thing that stems from the punch card ages, with a fixed-width format. Unfortunately, at some point that format changed, and things moved around. And there's a ton of programs out there that produce GenBank files that are slightly off. So parsing the header using a token-based approach seems like a good thing. Now, let's look at the first line. That has a bunch of interesting information.

LOCUS       SCU49845     5028 bp    DNA             PLN       21-JUN-1999

The 'LOCUS' tag explains what this line is about, then there's the accession number for the sequence, the length of the sequence, the type of the molecule, some classification where the sequence comes from, and the date the sequence last saw a major update. The regex to parse this is pretty straightforward (yeah, right):

LOCUS\s+([\w.]+)\s+(\d+)\sbp\s(ss-|ds-|ms-|\s{3,3})(\S{2,4})\s+(linear\s\s|circular|\s{7,7})\s+(\w{3,3})\s+(\d\d-\w{3,3}-\d{4,4})

Incidently, this is a great example for why people dislike regular expressions. Now, in both Perl and Python, there's a way to define verbose regular expressions, so you can restate the regular expression as:

LOCUS\s+        # Header line starts with LOCUS tag followed by multiple spaces
(               # accession number regex:
  [\w.]+        # any alphanumeric character or '.'
)
\s+             # skip over whitespace
(               # sequence length
  \d+           # digits only
)\sbp\s         # skip ' bp ' string
(               # single, double or mixed stranded or nothing
  ss-|ds-|ms-|\s\s\s # can be all spaces
)
(               # Molecule type DNA, RNA, rRNA, mRNA, uRNA
  \S+
)
\s+
(               # linear, circular or seven spaces
  linear|circular|\s{7,7}
)\s+
(               # division code, three characters
 \w{3,3}
)
\s+
(               # date, in dd-MMM-yyyy
  \d\d-\w{3,3}-\d{4,4}
)

This is already pretty decent, but Python can do one better. With the current version of the regex, I need to remember that the molecule type is the 4th group, so match.group(3) will be what I'm looking for. Python decided to extend the Perl extension syntax a bit more, to add named groups. With ?P<name> you can name groups. and then call match.group('name') to access them later. So the final version of the parser regex turns into

LOCUS\s+        # Header line starts with LOCUS tag followed by multiple spaces
(?P<accession>  # accession number regex:
  [\w.]+        # any alphanumeric character or '.'
)
\s+             # skip over whitespace
(?P<length>     # sequence length
  \d+           # digits only
)\sbp\s         # skip ' bp ' string
(?P<stranded>   # single, double or mixed stranded or nothing
  ss-|ds-|ms-|\s\s\s # can be all spaces
)
(?P<molecule>   # Molecule type DNA, RNA, rRNA, mRNA, uRNA
  \S+
)
\s+
(?P<formation>  # linear, circular or seven spaces
  linear|circular|\s{7,7}
)\s+
(?P<division>   # division code, three characters
 \w{3,3}
)
\s+
(?P<date>       # date, in dd-MMM-yyyy
  \d\d-\w{3,3}-\d{4,4}
)

and you can use speaking names to access the group's contents later. Great to make the code more readable. Combined with a bunch of tests, that should stay maintainable.