regex101: FEN Regex

^start of string

Comment: Piece Placement Section

Group PiecePlacement(?<PiecePlacement>((?<RankItem>[pnbrqkPNBRQK1-8]{1,8})\/?){8})

Group 2((?<RankItem>[pnbrqkPNBRQK1-8]{1,8})\/?){8}

{8}repeats the group contents below exactly 8 times:

Group RankItem(?<RankItem>[pnbrqkPNBRQK1-8]{1,8})

Character Class[pnbrqkPNBRQK1-8]{1,8}

{1,8}matches between 1 and 8 characters from the set below:

pthe literal p (112₁₀ / 160₈ / 70₁₆) (case sensitive)

nthe literal n (110₁₀ / 156₈ / 6E₁₆) (case sensitive)

bthe literal b (98₁₀ / 142₈ / 62₁₆) (case sensitive)

rthe literal r (114₁₀ / 162₈ / 72₁₆) (case sensitive)

qthe literal q (113₁₀ / 161₈ / 71₁₆) (case sensitive)

kthe literal k (107₁₀ / 153₈ / 6B₁₆) (case sensitive)

Pthe literal P (80₁₀ / 120₈ / 50₁₆) (case sensitive)

Nthe literal N (78₁₀ / 116₈ / 4E₁₆) (case sensitive)

Bthe literal B (66₁₀ / 102₈ / 42₁₆) (case sensitive)

Rthe literal R (82₁₀ / 122₈ / 52₁₆) (case sensitive)

Qthe literal Q (81₁₀ / 121₈ / 51₁₆) (case sensitive)

Kthe literal K (75₁₀ / 113₈ / 4B₁₆) (case sensitive)

1-8one character from 1 (49₁₀) to 8 (56₁₀)

\/?optionally matches the literal / (47₁₀ / 57₈ / 2F₁₆)

Comment: Section Separator

\s+any whitespace character, repeated at least once

Comment: Side To Move Section

Group SideToMove(?<SideToMove>b|w)

1st Alternativeb

bthe literal b (98₁₀ / 142₈ / 62₁₆) (case sensitive)

2nd Alternativew

wthe literal w (119₁₀ / 167₈ / 77₁₆) (case sensitive)

Comment: Section Separator

\s+any whitespace character, repeated at least once

Comment: Castling Ability

Group Castling(?<Castling>-|K?Q?k?q)

1st Alternative-

-the literal - (45₁₀ / 55₈ / 2D₁₆)

2nd AlternativeK?Q?k?q

K?optionally matches the literal K (75₁₀ / 113₈ / 4B₁₆) (case sensitive)

Q?optionally matches the literal Q (81₁₀ / 121₈ / 51₁₆) (case sensitive)

k?optionally matches the literal k (107₁₀ / 153₈ / 6B₁₆) (case sensitive)

qthe literal q (113₁₀ / 161₈ / 71₁₆) (case sensitive)

Comment: Section Separator

\s+any whitespace character, repeated at least once

Comment: En passant

Group EnPassant(?<EnPassant>-|[a-h][3-6])

1st Alternative-

-the literal - (45₁₀ / 55₈ / 2D₁₆)

2nd Alternative[a-h][3-6]

Character Class[a-h]

a-hone character from a (97₁₀) to h (104₁₀) (case sensitive)

Character Class[3-6]

3-6one character from 3 (51₁₀) to 6 (54₁₀)

Comment: Section Separator

\s+any whitespace character, repeated at least once

Comment: Half Move Clock

Group HalfMoveClock(?<HalfMoveClock>\d+)

\d+a digit, repeated at least once

Comment: Section Separator

\s+any whitespace character, repeated at least once

Comment: Full Move Number

Group FullMoveNumber(?<FullMoveNumber>\d+)

\d+a digit, repeated at least once

\s*any whitespace character, repeated any number of times

$end of string, or before its final line terminator

Global pattern flags

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

Search reference

Regular Expression

^
(?# Piece Placement Section)(?<PiecePlacement>((?<RankItem>[pnbrqkPNBRQK1-8]{1,8})\/?){8})
(?# Section Separator)\s+
(?# Side To Move Section)   (?<SideToMove>b|w)
(?# Section Separator)\s+
(?# Castling Ability)       (?<Castling>-|K?Q?k?q)
(?# Section Separator)\s+
(?# En passant)             (?<EnPassant>-|[a-h][3-6])
(?# Section Separator)\s+
(?# Half Move Clock)        (?<HalfMoveClock>\d+)
(?# Section Separator)\s+
(?# Full Move Number)       (?<FullMoveNumber>\d+)
\s*
$

New

rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq - 1 2