I am a newbie in Python. I want to write a regular expression for some name checking. My input string can contain a-z, A-Z, 0-9, and ' _ ', but it should start with either a-z or A-Z (not 0-9 and ' _ '). I want to write a regular expression for this. I tried, but nothing was matching perfectly.

Once the input string follows the regular expression rules, I can proceed further, otherwise discard that string.

Here's an answer to your question:

Interpreting that you want _ (not -), this should do the job:

>>> tests = ["a", "A", "a1", "a_1", "1a", "_a", "a\n", "", "z_"]
>>> for test in tests:
...    print repr(test), bool(re.match(r"[A-Za-z]\w*\Z", test))
...
'a' True
'A' True
'a1' True
'a_1' True
'1a' False
'_a' False
'a\n' False
'' False
'z_' True
>>>

Stoutly resist the temptation to use $; here's why:

Hello, hello, using $ is WRONG, use \Z instead

>>> re.match(r"[a-zA-Z][\w-]*$","A")
<_sre.SRE_Match object at 0x00BAFE90>
>>> re.match(r"[a-zA-Z][\w-]*$","A\n")
<_sre.SRE_Match object at 0x00BAFF70> # WRONG; SHOULDN'T MATCH
>>>>>> re.match(r"[a-zA-Z][\w-]*\Z","A")
<_sre.SRE_Match object at 0x00BAFE90>
>>> re.match(r"[a-zA-Z][\w-]*\Z","A\n")
>>> # CORRECT: NO MATCH

The Fine Manual says:

'$'
Matches the end of the string or just before the newline at the end of the string [my emphasis], and in MULTILINE mode also matches before a newline. foo matches both ‘foo’ and ‘foobar’, while the regular expression foo$ matches only ‘foo’. More interestingly, searching for foo.$ in 'foo1\nfoo2\n' matches ‘foo2’ normally, but ‘foo1’ in MULTILINE mode; searching for a single $ in 'foo\n' will find two (empty) matches: one just before the newline, and one at the end of the string.

and

\Z
Matches only at the end of the string.

=== And now for something completely different ===

>>> import string
>>> letters = set(string.ascii_letters)
>>> ok_chars = letters | set(string.digits + "_")
>>>
>>> def is_valid_name(strg):
...     return strg and strg[0] in letters and all(c in ok_chars for c in strg)
...
>>> for test in tests:
...     print repr(test), repr(is_valid_name(test))
...
'a' True
'A' True
'a1' True
'a_1' True
'1a' False
'_a' False
'a\n' False
'' ''
'z_' True
>>>