expr: incorrectly handle regexp and breaks configure scripts

The PR #7842 changed the way expr handle regexp, breaking configure scripts created by autotools.

```
#export C=coreutils/target/debug/

${C}expr xstatic : '.*[^-+._abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789]'    #0
```