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22.2.8 * versus .*

This section compares file globbing with regular expression matching. There are many Unix commands which are regularly used from shell scripts, and which provide some sort of pattern matching mechanism: expr, egrep and sed, to name a few. Unfortunately they each have different quoting rules regarding whether particular meta-characters must be backslash escaped to revert to their literal meaning and vice-versa. There is no real logic to the particular dialect of regular expressions accepted by these commands. To confirm the correctness of each regular expression, you should always check them from the shell prompt with the relevant tool before committing to a script, so I won't belabour the specifics.

Shell globbing however is much more regular (no pun intended), and provides a reasonable and sometimes more cpu efficient solution to many shell matching problems. The key is to make good use of the case command, which is easier to use (because it uses globbing rules) and doesn't require additional processes to be spawned. Unfortunately, GNU Bash doesn't handle backslashes correctly in glob character classes -- the backslash must be the first character in the class, or else it will never match. For example, if you want to detect absolute directory paths on Unix and Windows using case, you should write the code like this:

case $dir in
  [\\/]* | ?:[\\/]* ) echo absolute ;;
  * )                 echo relative ;;

Even though expr uses regular expressions rather than shell globbing, it is often(52) a shell builtin, so using it to extract sections of strings can be faster than spawning a sed process to do the same. As with echo and set, for example, you must be careful that variable or command expansions for the first argument to expr are not accidentally interpreted as reserved keywords. As with echo, you can work around this problem by prefixing any expansions with a literal `x', as follows:

$ foo=substr
$ expr $foo : '.*\(str\)'
expr: syntax error
$ expr x$foo : '.*\(str\)'

This document was generated by Gary V. Vaughan on February, 8 2006 using texi2html