Residue ID key error if _ in the chain

[yuyuan871111]

test pdb: receptor.txt (Github is not allowed to upload pdb, so I modified it into txt.)

The water molecules are with a chain "_".

TER    3277      PHE A 456                                                      
HETATM 3278  O   HOH _   1      26.096  36.294  30.269  1.00 32.18           O  
HETATM 3279  O   HOH _   2      23.309  35.382  24.389  1.00 23.49           O  
HETATM 3280  O   HOH _   3      30.682  28.763  30.563  1.00 22.28           O  
HETATM 3281  O   HOH _   4      23.773  31.116  21.198  1.00 16.56           O  
HETATM 3282  O   HOH _   5      27.333  37.316  26.228  1.00 32.51           O  
HETATM 3283  O   HOH _   6      28.153  35.475  19.899  1.00 39.46           O  
HETATM 3284  O   HOH _   7      26.977  32.208  19.549  1.00 33.98           O  
END

When reading a file, the resid is shown as ResidueId(HOH, 7, _).

import MDAnalysis as mda

protein_mol = Molecule.from_rdkit(Chem.MolFromPDBFile(test_file))
protein_mol.residues[-1].resid

>>> ResidueId(HOH, 7, _)

However, when I tried to find this residue using a string, there was a key error.

protein_mol["HOH7._"]

>>> KeyError: ResidueId(HOH, 7, None)

When I checked with ResidueID class, I found it is attributed to the limitation of the regex in from_string function.

ProLIF/prolif/residue.py

Line 22 in 185460c

r"(TIP[234]|T[234]P|H2O|[0-9][A-Z]{2}|[A-Z ]+)?(\d*)\.?([A-Z\d]{1,2})?"

Here is my test:

from prolif.residue import ResidueId

ResidueId.from_string("HOH7._")

>>> KeyError: ResidueId(HOH, 7, None)

Original regex:
r"(TIP[234]|T[234]P|H2O|[0-9][A-Z]{2}|[A-Z ]+)?(\d*)\.?([A-Z\d]{1,2})?"

I suggest to have "_" in the group 3.
Modifled regex:
r"(TIP[234]|T[234]P|H2O|[0-9][A-Z]{2}|[A-Z ]+)?(\d*)\.?([A-Z_\d]{1,2})?"