Regular expression - starting and ending with a letter accepting only letters numbers and

Question

I m trying to write a regular expression which specifies that text should start with a letter  every character should be a letter  number or underscore  there should not be 2 underscores in a row and it should end with a letter or number  At the moment  the only thing I have is   a-zA-Z  w a-zA-Z1-9   but this doesn t seem to work properly since it only ever matches 3 characters  and allows repeated underscores  I also don t know how to specify requirements for the last character

User · Answer

I ll take a stab at it      a-z       a-z0-9      i   Explained                    match beginning of string   a-z          match a letter for the first char                start non-capture group                  match 0 or 1         a-z0-9       match a letter or number  1 or more times                end non-capture group  match whole group 0 or more times                match end of string  i             case insensitive flag   The non-capture group takes care of a  not allowing two   s  it forces at least one letter or number per group  and b  only allowing the last char to be a letter or number   Some test strings    a   match      fail  zz   match  a0   match  A    fail  a0 b   match  a  b   fail  a 1 c   match

User · Answer

A-Za-z  A-Za-z0-9       A-Za-z0-9

User · Answer

Here s a solution using a negative lookahead  not supported in all regex engines      a-zA-Z          a-zA-Z0-9     a-zA-Z0-9       Test that it works as expected   import re tests          a   True            False        zz   True        a0   True        A    False        a0 b   True        a  b   False        a 1 c   True      regex      a-zA-Z          a-zA-Z0-9     a-zA-Z0-9      for test in tests     is match   re match regex  test 0   is not None    if is match    test 1          print  fail       test 0

User · Answer

seeing how the rules are fairly complicated  I d suggest the following      a-z   w   a-z0-9   i   match the whole string and capture intermediate characters  Then either with the string functions or the following regex          check if the captured part has two underscores in a row  For example in Python it would look like this    gt  gt  gt  import re  gt  gt  gt  def valid s       match   re match r   a-z   w   a-z0-9     s  re I      if match is not None          return match group 1  count          0     return False

[regex] Regular expression - starting and ending with a letter, accepting only letters, numbers and _

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