Improve regex tokenizer example by using re.finditer().

Also, improve variable names and fix column numbers
in the generated output.

Doc/library/re.rst

@@ -1333,7 +1333,7 @@ successive matches::
 
     Token = collections.namedtuple('Token', ['typ', 'value', 'line', 'column'])
 
-    def tokenize(s):
+    def tokenize(code):
         keywords = {'IF', 'THEN', 'ENDIF', 'FOR', 'NEXT', 'GOSUB', 'RETURN'}
         token_specification = [
             ('NUMBER',  r'\d+(\.\d*)?'), # Integer or decimal number
@@ -1343,26 +1343,27 @@ successive matches::
             ('OP',      r'[+\-*/]'),     # Arithmetic operators
             ('NEWLINE', r'\n'),          # Line endings
             ('SKIP',    r'[ \t]+'),      # Skip over spaces and tabs
+            ('MISMATCH',r'.'),           # Any other character
         ]
         tok_regex = '|'.join('(?P<%s>%s)' % pair for pair in token_specification)
         get_token = re.compile(tok_regex).match
-        line = 1
-        pos = line_start = 0
-        mo = get_token(s)
-        while mo is not None:
-            typ = mo.lastgroup
-            if typ == 'NEWLINE':
-                line_start = pos
-                line += 1
-            elif typ != 'SKIP':
-                val = mo.group(typ)
-                if typ == 'ID' and val in keywords:
-                    typ = val
-                yield Token(typ, val, line, mo.start()-line_start)
-            pos = mo.end()
-            mo = get_token(s, pos)
-        if pos != len(s):
-            raise RuntimeError('Unexpected character %r on line %d' %(s[pos], line))
+        line_num = 1
+        line_start = 0
+        for mo in re.finditer(tok_regex, code):
+            kind = mo.lastgroup
+            value = mo.group(kind)
+            if kind == 'NEWLINE':
+                line_start = mo.end()
+                line_num += 1
+            elif kind == 'SKIP':
+                pass
+            elif kind == 'MISMATCH':
+                raise RuntimeError('%r unexpected on line %d' % (value, line_num))
+            else:
+                if kind == 'ID' and value in keywords:
+                    kind = value
+                column = mo.start() - line_start
+                yield Token(kind, value, line_num, column)
 
     statements = '''
         IF quantity THEN
@@ -1376,22 +1377,22 @@ successive matches::
 
 The tokenizer produces the following output::
 
-    Token(typ='IF', value='IF', line=2, column=5)
-    Token(typ='ID', value='quantity', line=2, column=8)
-    Token(typ='THEN', value='THEN', line=2, column=17)
-    Token(typ='ID', value='total', line=3, column=9)
-    Token(typ='ASSIGN', value=':=', line=3, column=15)
-    Token(typ='ID', value='total', line=3, column=18)
-    Token(typ='OP', value='+', line=3, column=24)
-    Token(typ='ID', value='price', line=3, column=26)
-    Token(typ='OP', value='*', line=3, column=32)
-    Token(typ='ID', value='quantity', line=3, column=34)
-    Token(typ='END', value=';', line=3, column=42)
-    Token(typ='ID', value='tax', line=4, column=9)
-    Token(typ='ASSIGN', value=':=', line=4, column=13)
-    Token(typ='ID', value='price', line=4, column=16)
-    Token(typ='OP', value='*', line=4, column=22)
-    Token(typ='NUMBER', value='0.05', line=4, column=24)
-    Token(typ='END', value=';', line=4, column=28)
-    Token(typ='ENDIF', value='ENDIF', line=5, column=5)
-    Token(typ='END', value=';', line=5, column=10)
+    Token(typ='IF', value='IF', line=2, column=4)
+    Token(typ='ID', value='quantity', line=2, column=7)
+    Token(typ='THEN', value='THEN', line=2, column=16)
+    Token(typ='ID', value='total', line=3, column=8)
+    Token(typ='ASSIGN', value=':=', line=3, column=14)
+    Token(typ='ID', value='total', line=3, column=17)
+    Token(typ='OP', value='+', line=3, column=23)
+    Token(typ='ID', value='price', line=3, column=25)
+    Token(typ='OP', value='*', line=3, column=31)
+    Token(typ='ID', value='quantity', line=3, column=33)
+    Token(typ='END', value=';', line=3, column=41)
+    Token(typ='ID', value='tax', line=4, column=8)
+    Token(typ='ASSIGN', value=':=', line=4, column=12)
+    Token(typ='ID', value='price', line=4, column=15)
+    Token(typ='OP', value='*', line=4, column=21)
+    Token(typ='NUMBER', value='0.05', line=4, column=23)
+    Token(typ='END', value=';', line=4, column=27)
+    Token(typ='ENDIF', value='ENDIF', line=5, column=4)
+    Token(typ='END', value=';', line=5, column=9)