s=>!/^d|W|^$/.test(s)/-~(s[0]=='_')

¬D'_Qsa·+¹žj-""Q*2/

¬                     # Push input and also push the first character.
 D                    # Duplicate the first character.
  '_Q                 # Check if it is equal to an underscore character.
     sa               # Swap and check the duplicate if it's an alphabetic character.
       ·              # Double the value.
        +             # Add both values up
         ¹            # Take the first input.
          žj-         # žj is short for [a-zA-Z0-9_]. This will be substracted from the
                        initial string.
             ""Q      # Check if the string is empty.
                *     # Multiply this with the first value.
                 2/   # Halve it, resulting into 0.0, 0.5, or 1.0.

((s[0] == '_' + s.isalpha() × 2) × (s.remove([a-zA-Z0-9_]) == "")) / 2

preg_match('/^[a-z_]w*$/i',$s)?$s[0]=='_'?.5:1:0;

preg_match('/^[a-z_]w*$/i', $s) # Matches every a-zA-Z0-9_ string that doesnt start with a number
?   $s[0] == '_'                   # Then, if it starts with an _
    ?   .5                         # give 0.5 points
    :   1                          # If it doesn't, give 1
:   0;                             # If it didn't match the regex, give 0

_test_

a-z

1)95=2/8M3Y2m+G7M95h4Y2hmA*

1)      % take input implicitly. Get its first element
95=     % true if it equals 95 (underscore)
2/      % divide by 2: gives 0.5 if underscore, 0 if not
8M      % push first element of input again
3Y2     % predefined literal: string with all letters
m       % true if it's a letter
+       % add. Gives 1 if letter, 0.5 if underscore
G       % push input again
7M      % push string with all letters again
95h     % concatenate underscore
4Y2h    % predefined literal: string with all digits. Concatenate
mA      % true if all input chars belong to that concatenated string
*       % multiply. Display implicitly

Qh~u{Q~J_+-|!Qh_qh/

Qh~u{                 - Check first char isn't a digit
     Q~J_+-          - Is the input alphanumeric + "_"
            |!        - Combine
              Qh_q   - Is the first char an "_"
                   h/ - Combine

lambda s:s.isidentifier()/-~(s[:1]=='_')

c!|-rz0++G_JjkUT}hzJhqhz_

÷"[^Wd]w*"g¦"_.*"g+÷2=0.5¿

$ ./gogh no '÷"[^Wd]w*"g¦"_.*"g+÷2=0.5¿' "_test"

“ Implicit input                               ”
÷                  “ Duplicate the TOS                            ”
"[^Wd]w*"g      “ Fully match the STOS against the TOS (regex) ”
¦                  “ Swap the STOS and TOS                        ”
"_.*"g             “ Fully match the STOS against the TOS (regex) ”
+                  “ Add the TOS to the STOS                      ”
÷                  “ Duplicate the TOS                            ”
2=                 “ Determine if the TOS is equal to 2           ”
0.5¿               “ Leave the correct output on the stack        ”
                   “ Implicit output                              ”

$_=!/W|^d//2**/^_/

c!:z"W|^d"0h!xz_

c!:z"W|^d"0h!xz_  (implicit) Save the input in z.

  :z        0        Test if z matches the following regex:
    "W|^d"           A non-word character or a digit at the beginning.
                     This returns True iff z is an invalid name.
 !                   Apply logical NOT to yield True iff z is a valid name.
               xz_  Find the first index of the underscore in z.
                     This yields 0 iff z begins with an underscore.
             h!      Apply logical NOT and increment.
                     This yields 2 if z begins with an underscore, 1 otherwise.
c                    Divide the two results.

USE: regexp
[ R/ [_a-zA-Z]w*/ R/ _.*/ [ matches? 1 0 ? ] bi-curry@ bi 0 = 1 2 ? / ]

USE: regexp
: v ( s -- n ) R/ [_a-zA-Z]w*/ R/ _.*/ [ matches? 1 0 ? ] bi-curry@ bi 0 = 1 2 ? / ;

{ 0 } [ "" v ] unit-test
{ 0 } [ "" v ] unit-test
{ 0 } [ "1" v ] unit-test
{ 0 } [ "1var" v ] unit-test
{ 0 } [ "var$" v ] unit-test
{ 0 } [ "foo var" v ] unit-test
{ 1 } [ "v" v ] unit-test
{ 1 } [ "var" v ] unit-test
{ 1 } [ "var_i_able" v ] unit-test
{ 1 } [ "v4r14bl3" v ] unit-test
{ 1/2 } [ "_" v ] unit-test
{ 1/2 } [ "_v" v ] unit-test
{ 1/2 } [ "_var" v ] unit-test
{ 1/2 } [ "_var_i_able" v ] unit-test
{ 1/2 } [ "_v4r14bl3" v ] unit-test

{(0≤⎕NC⍵)÷1+'_'=⊃⍵}

If[#~StringMatchQ~RegularExpression@"[A-Za-z_][0-9A-Za-z_]*",If[#~StringTake~1=="_",.5,1],0]&

$_=/^([^Wd])w*$//(($1 eq"_")+1)

$_=/^([^Wd])w*$//(($1 eq"_")+1)
   /^([^Wd])w*$/                 matches any string that starts with an underscore or a letter of the alphabet followed by 0 or more alphanumeric + underscore characters. The first character is stored in a capture group
                   /                divide result by
                    (($1 eq"_")+1)  (capture == "_") + 1. This is 1 if the first character was not an underscore and 2 if it was.
$_=                                 assign to $_ and implicitly print

lambda x:[0,[[.5,1][x[0]>'z'],0][x[0]<'A']][x.replace('_','a').isalnum()]if x else 0

x=>!x.match(/W|^d/)/2**/^_/.test(x)

x=>                                   // Fat arrow function
   !x.match(/W|^d/)                 // Gives false if contains non word or starting
                                      //   with a digit. Booleans in numeric context will
                                      //   be 0 or 1
                      2**             // 2 to the power of...
                         /^_/.test(x) // gives true if starting with '_'.
                                      //   true -> 1 -> 2**1 -> 2
                                      //   false -> 0 -> 2**0 -> 1
                     /                // Devide the lValue boolean with the numeric rValue:
                                      // lValue = 0 or 1
                                      // rValue = 2 or 1

->(s){s=~/^(_|d)?w*$/?$1?$1==?_?0.5:0:1:0}

->(s){case s
when'',/^d/,/W/
0
when/^_/
0.5
else
1
end}

v=function(s)return(s:match("^[_%a]+[_%w]*$")and 1or 0)*(s:match("_")and.5or 1)end

print(v("a") == 1) -- true
print(v("1") == 0) -- true
print(v("_") == 0.5) -- true
print(v("") == 0) -- true
print(v("1a") == 0) -- true
print(v("a1") == 1) -- true
print(v("_1") == 0.5) -- true
print(v("_a") == 0.5) -- true
print(v("1_") == 0) -- true
print(v("a_") == 0.5) -- true

s=arg[1]print(s:find("^[%a_][%w_]*$")and(s:find("^_")and.5or 1)or 0)