正如评论中所指出的，expr 只使用basic"；(又名过时")正则表达式，并且 bash 的 =~ 运算符使用的正则表达式引擎不支持前瞻或后视(或 s 空格的简写)).但是你不需要环顾四周，只需匹配一切并使用捕获组来挑选你想要的部分(并将模式存储在一个变量中以避免可能的解析不一致):

As was pointed out in the comments, expr only uses "basic" (aka "obsolete") regular expressions, and the regex engine that bash's =~ operator uses doesn't support lookahead or lookbehind (or the s shorthand for space either). But you don't need lookaround, just match everything and use a capture group to pick out the part you want (and store the pattern in a variable to avoid possible parsing inconsistencies):

WorkingDirPattern='"WorkingDir":[[:space:]]"([^"]+)"'
if [[ "$config" =~ $WorkingDirPattern ]]; then
    WorkingDir="${BASH_REMATCH[1]}"    # get the contents of the first capture group
    echo "WorkingDir is $WorkingDir"
else
    echo "No WorkingDir found" >&2
fi