diff --git a/pubs/commands/add_cmd.py b/pubs/commands/add_cmd.py
index f17a662..a425434 100644
--- a/pubs/commands/add_cmd.py
+++ b/pubs/commands/add_cmd.py
@@ -10,14 +10,14 @@ from .. import color
 from .. import pretty
 from .. import utils
 
+
 class ValidateDOI(argparse.Action):
     def __call__(self, parser, namespace, values, option_string=None):
         doi = values
         new_doi = utils.standardize_doi(doi)
-        if not new_doi:
-            raise ValueError("Not a valid doi: %s", doi)
         setattr(namespace, self.dest, new_doi)
 
+
 def parser(subparsers, conf):
     parser = subparsers.add_parser('add', help='add a paper to the repository')
     parser.add_argument('bibfile', nargs='?', default=None,
@@ -30,9 +30,9 @@ def parser(subparsers, conf):
     parser.add_argument('-k', '--citekey', help='citekey associated with the paper;\nif not provided, one will be generated automatically.',
                         default=None)
     parser.add_argument('-L', '--link', action='store_false', dest='copy', default=True,
-            help="don't copy document files, just create a link.")
+                        help="don't copy document files, just create a link.")
     parser.add_argument('-M', '--move', action='store_true', dest='move', default=False,
-            help="move document instead of of copying (ignored if --link).")
+                        help="move document instead of of copying (ignored if --link).")
     return parser
 
 
diff --git a/pubs/utils.py b/pubs/utils.py
index bbfeab5..60aadac 100644
--- a/pubs/utils.py
+++ b/pubs/utils.py
@@ -1,10 +1,10 @@
 # Function here may belong somewhere else. In the mean time...
-
 import re
 
 from . import color
 from . import pretty
 
+
 def resolve_citekey(repo, citekey, ui=None, exit_on_fail=True):
     """Check that a citekey exists, or autocompletes it if not ambiguous."""
     """ :returns found citekey """
@@ -47,6 +47,7 @@ def resolve_citekey_list(repo, citekeys, ui=None, exit_on_fail=True):
     else:
         return keys
 
+
 def standardize_doi(doi):
     """
     Given a putative doi, attempts to always return it in the form of
@@ -60,22 +61,22 @@ def standardize_doi(doi):
     and attempts to verify doi adherence to DOI handbook standards and
     crossref.org advice:
     https://www.doi.org/doi_handbook/2_Numbering.html
-    https://www.crossref.org/blog/dois-and-matching-regular-expressions/"""
-    """ :returns standardized doi """
+    https://www.crossref.org/blog/dois-and-matching-regular-expressions/
+
+        :returns standardized doi
+    """
+
     doi_regexes = (
-        re.compile(r'(10\.\d{4,9}/[-._;()/:A-z0-9\>\<]+)'),
-        re.compile(r'(10.1002/[^\s]+)'),
-        re.compile(r'(10\.\d{4}/\d+-\d+X?(\d+)\d+<[\d\w]+:[\d\w]*>\d+.\d+.\w+;\d)'),
-        re.compile(r'(10\.1021/\w\w\d+\+)'),
-        re.compile(r'(10\.1207/[\w\d]+\&\d+_\d+)')
-        )
+        '(10\.\d{4,9}/[-._;()/:A-z0-9\>\<]+)',
+        '(10.1002/[^\s]+)',
+        '(10\.\d{4}/\d+-\d+X?(\d+)\d+<[\d\w]+:[\d\w]*>\d+.\d+.\w+;\d)',
+        '(10\.1021/\w\w\d+\+)',
+        '(10\.1207/[\w\d]+\&\d+_\d+)')
+    doi_pattern = re.compile('|'.join(doi_regexes))
 
-    for doi_regex in doi_regexes:
-        match = doi_regex.search(doi)
-        if match:
-            new_doi = match.group(0)
-            break
-    else:
-        new_doi = None
+    match = doi_pattern.search(doi)
+    if not match:
+        raise ValueError("Not a valid doi: %s", doi)
+    new_doi = match.group(0)
 
     return new_doi