以下是我的做法:

FN=someletters_12345_moreleters.ext
[[ ${FN} =~ _([[:digit:]]{5})_ ]] && NUM=${BASH_REMATCH[1]}

解释:

Bash-specific:

[[]]为条件表达式 =~表示条件为正则表达式 如果前一个命令成功，&&将链接这些命令

正则表达式(RE): _([[:digit:]]{5})_

_是字面量，用于为被匹配的字符串划分/锚定匹配边界 ()创建捕获组 [[:digit:]]是一个字符类，我认为它不言自明 {5}表示前面的字符中的恰好五个，类(如本例中所示)或组必须匹配

In english, you can think of it behaving like this: the FN string is iterated character by character until we see an _ at which point the capture group is opened and we attempt to match five digits. If that matching is successful to this point, the capture group saves the five digits traversed. If the next character is an _, the condition is successful, the capture group is made available in BASH_REMATCH, and the next NUM= statement can execute. If any part of the matching fails, saved details are disposed of and character by character processing continues after the _. e.g. if FN where _1 _12 _123 _1234 _12345_, there would be four false starts before it found a match.

很多过时的解决方案都需要管道和子外壳。 自bash版本3(2004年发布)以来，它有一个内置的正则表达式比较操作符=~。

input="someletters_12345_moreleters.ext"
# match: underscore followed by 1 or more digits followed by underscore
[[ $input =~ _([0-9]+)_ ]]
echo ${BASH_REMATCH[1]}

输出:

12345

注意，如果您不是很精通编写RegExp，我建议您阅读精通正则表达式。

如果您只是需要弄清楚如何让RegExp工作，并且它不符合您的想法，请尝试RegEx101.com的在线GUI，并将“Flavor”设置为“PCRE”，以便获得bash使用的[[:digit:]]等POSIX风格的字符类。

基于jor的回答(这对我来说并不适用):

substring=$(expr "$filename" : '.*_\([^_]*\)_.*')

使用sed replace的简单方法:

result=$(echo "someletters_12345_moreleters.ext" | sed 's/.*_\(.*\)_.*/\1/g')
echo $result

还有bash内置的'expr'命令:

INPUT="someletters_12345_moreleters.ext"  
SUBSTRING=`expr match "$INPUT" '.*_\([[:digit:]]*\)_.*' `  
echo $SUBSTRING

Inklusive end，类似于JS和Java实现。删除+1如果你不想这样做。

function substring() {
    local str="$1" start="${2}" end="${3}"
    
    if [[ "$start" == "" ]]; then start="0"; fi
    if [[ "$end"   == "" ]]; then end="${#str}"; fi
    
    local length="((${end}-${start}+1))"
    
    echo "${str:${start}:${length}}"
} 

例子:

    substring 01234 0
    01234
    substring 012345 0
    012345
    substring 012345 0 0
    0
    substring 012345 1 1
    1
    substring 012345 1 2
    12
    substring 012345 0 1
    01
    substring 012345 0 2
    012
    substring 012345 0 3
    0123
    substring 012345 0 4
    01234
    substring 012345 0 5
    012345

更多示例调用:

    substring 012345 0
    012345
    substring 012345 1
    12345
    substring 012345 2
    2345
    substring 012345 3
    345
    substring 012345 4
    45
    substring 012345 5
    5
    substring 012345 6
    
    substring 012345 3 5
    345
    substring 012345 3 4
    34
    substring 012345 2 4
    234
    substring 012345 1 3
    123