我的答案将对你想从字符串中得到什么有更多的控制。下面是如何从字符串中提取12345的代码

str="someletters_12345_moreleters.ext"
str=${str#*_}
str=${str%_more*}
echo $str

如果你想提取像abc这样的字符或像_或-这样的特殊字符，这样会更有效。例如:如果你的字符串是这样的，你想要someletters_之后和_moreleters之前的所有内容。ext:

str="someletters_123-45-24a&13b-1_moreleters.ext"

使用我的代码，您可以确切地说出您想要什么。 解释:

#*它将删除前面的字符串，包括匹配的键。这里我们提到的键是_ 它将删除以下字符串，包括匹配的键。这里我们提到的键是_more*

自己做一些实验，你会发现这很有趣。

基于jor的回答(这对我来说并不适用):

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

我很惊讶这个纯粹的bash解决方案没有出现:

a="someletters_12345_moreleters.ext"
IFS="_"
set $a
echo $2
# prints 12345

您可能希望将IFS重置为之前的值，或者在之后取消设置IFS !

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.

给定test.txt文件包含"ABCDEFGHIJKLMNOPQRSTUVWXYZ"

cut -b19-20 test.txt > test1.txt # This will extract chars 19 & 20 "ST" 
while read -r; do;
> x=$REPLY
> done < test1.txt
echo $x
ST

这里是纯参数替换，一个空字符串。注意，我只将一些字母和更多字母定义为字符。如果它们是字母数字，这将无法正常工作。

filename=someletters_12345_moreletters.ext
substring=${filename//@(+([a-z])_|_+([a-z]).*)}
echo $substring
12345