Is this string valid FEN?
JavaScript (ES6), 168 174 ... 155
This answer has been edited an embarrassing number of times. Hopefully, the current version is both reliable and decently golfed.
Returns a boolean.
s=>[...s].map(c=>++n%9?+c?n+=--c:a[i='pP/KkQqRrBbNn'.search(c),i&=i>4&a[i]>(i>6)||i]=-~a[i]:x+=c=='/',a=[x=n=0])&&!([p,P,s,k,K]=a,n-71|x-7|s|k*K-1|p>8|P>8)
Formatted and commented
s => [...s].map(c => // for each character 'c' in the FEN string 's':
++n % 9 ? // if we haven't reached the end of a rank:
+c ? // if the character is a digit:
n += --c // advance the board pointer by c - 1 squares
: // else:
a[ // update the piece counter array:
i = // i = piece identifier (0 to 12)
'pP/KkQqRrBbNn'.search(c), // with special case: '/' --> 2
i &= // we count it as a promoted pawn instead if:
i > 4 & // it's a Q, R, B or N and we already have
a[i] > (i > 6) || // 2 of them for R, B, N or just 1 for Q
i // else, we keep the identifier unchanged
] = -~a[i] // '-~' allows to increment 'undefined'
: // else:
x += c == '/', // check that the expected '/' is there
a = [ // initialize the piece counter array 'a'
x = // initialize the '/' counter 'x',
n = 0 ] // initialize the board pointer 'n'
) && // end of map()
!( // now it's time to perform all sanity checks:
[p, P, s, K, k] = a, // copy the 5 first entries of 'a' to new variables
n - 71 | // have we reached exactly the end of the board?
x - 7 | // have we identified exactly 7 ends of rank?
s | // have we encountered any unexpected '/' character?
k * K - 1 | // do we have exactly one king on each side?
p > 8 | // no more than 8 black pawns, including promotions?
P > 8) // no more than 8 white pawns, including promotions?
Test cases
let f =
s=>[...s].map(c=>++n%9?+c?n+=--c:a[i='pP/KkQqRrBbNn'.search(c),i&=i>4&a[i]>(i>6)||i]=-~a[i]:x+=c=='/',a=[x=n=0])&&!([p,P,s,k,K]=a,n-71|x-7|s|k*K-1|p>8|P>8)
console.log(f("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR")) // True
console.log(f("2br2k1/1p2n1q1/p2p2p1/P1bP1pNp/1BP2PnP/1Q1B2P1/8/3NR2K")) // True
console.log(f("r2r2k1/p3q2p/ppR3pr/rP4bp/3p4/5B1P/P4PP1/3Q1RK1")) // False (black has 7 pawns and 4 rooks - impossible)
console.log(f("6k1/pp3ppp/4p3/2P3b1/bPP3P1/3K4/P3Q1q1")) // False (only 7 ranks)
console.log(f("3r1rk1/1pp1bpp1/6p1/pP1npqPn/8/4N2P/P2PP3/1B2BP2/R2QK2R")) // False (9 ranks)
console.log(f("5n1k/1p3r1qp/p3p3/2p1N2Q/2P1R3/2P5/P2r1PP1/4R1K1")) // False (2nd rank has 9 squares/pieces)
console.log(f("rnbqkbnr/pppppppp/8/35/8/8/PPPPPPPP/RNBQKBNR")) // True
console.log(f("rnbqkbnr/pppppppp/8/9/7/8/PPPPPPPP/RNBQKBNR")) // False (additional test case suggested by Rick Hitchcock)
console.log(f("rnbqkbnr/pppp/ppp/8/8/8/8/PPPPPPPP/RNBQKBNR")) // False (additional test case)Retina, 105 bytes
[1-8]
$*
^
/
iG`^(/[1KQRBNP]{8}){8}$
G`K
G`k
A`K.*K|k.*k
{2`N
2`B
2`R
1`Q
K
T`L`P
8`P
A`P
}T`l`L
^.
Try it online! Link includes test cases. Explanation:
[1-8]
$*
Expand digits to empty squares, which we denote using 1s.
^
/
iG`^(/[1KQRBNP]{8}){8}$
Delete the input if it does not match 8 sets of 8 valid squares joined with /s. (An extra / is prefixed to simplify the check.)
G`K
G`k
A`K.*K|k.*k
Delete the input if it has no white or no black king, or if it has two of either.
{2`N
2`B
2`R
1`Q
K
Delete white's starting pieces, if they are still there.
T`L`P
Demote any remaining white pieces to pawns.
8`P
Delete the valid white pawns.
A`P
Delete the input if there are any white pawns left over.
}T`l`L
Check again but with the black pieces.
^.
Output a truthy value unless the line was deleted.
Python 3, 284 259 236 225 247 234 bytes
import re
s=input()
t,c=s.split("/"),s.count;P=p=9;o=0
for x in"pqrnb":p-=max(0,c(x)-o);P-=max(0,c(x.upper())-o);o+=o<2
v=8==len(t)and all(8==sum(int(x)for x in re.sub("[A-z]","1",p))for p in t)and p>0<P and c('k')==c('K')==1
print(v)Try it Online!
Try it Online with all of the test cases!
-11 bytes thanks to Mr. Xcoder
-13 bytes thanks to Jonathan Allen
+22 I forgot kings existed.
Semi-ungolfed with some explanation:
import re
string = input()
split = string.split("/")
count = string.count # find # of occurences of char in string
pawns = 9 # represents the # of pawns a player has out of the game... plus one, e.g. 1 is all in game, 2 is one out, 0 is invalid
PAWNS = 9 # would be 8, but then I would need >= instead of >
offset = 0 # default for pawns
for char in "pqrnb": # for each pawn, each queen over 1, and each rook/knight/bishop over 2 for each player
# subtract one from the players 'pawns' var, which must end up 1 or greater to be valid
# otherwise too many pawns/queens/etc of that player are on the board
pawns -= max(0,count(char)-offset)
PAWNS -= max(0,count(char.upper())-offset)
offset += (offset 0 and PAWNS>0 and \ # make sure each player does not have an invalid number of pawns/q/n/b/r
count('k')==count('K')==1 # correct # of kings
print(valid)