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Top Forums Shell Programming and Scripting compare variable against regular expression? Post 302198036 by era on Thursday 22nd of May 2008 06:20:17 AM
Old 05-22-2008
The if command simply evaluates another command, and examines its exit code to decide whether to take the then branch or the else branch (if present). You can use the expr command to match a string against a regular expression, or echo | grep. I would recommend case for this, though, even if it actually uses glob expressions, not true regular expressions.

Some shells also have built-in regex operators with the [[ conditional ]] syntax.

Code:
if echo "$var" | grep '[^0-9]'; then
  echo not a number  
fi
if expr "$var" : '[^0-9]' >/dev/null; then
  echo not a number
fi
case $var in
  *[!0-9]*) echo not a number;;    # not a regex proper
esac
if [[ "$var" =~ [^0-9] ]] ; then
  echo not a number
fi

All of these are more or less functionally equivalent. (They will not reject an empty string as not a number; implementing that is left as an exercise.)

Last edited by era; 05-22-2008 at 07:23 AM.. Reason: Will accept empty string as well as numbers, actually
 

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RE_COMP(3)						   BSD Library Functions Manual 						RE_COMP(3)

NAME
re_comp, re_exec -- regular expression handler LIBRARY
Compatibility Library (libcompat, -lcompat) SYNOPSIS
#include <unistd.h> char * re_comp(const char *s); int re_exec(const char *s); DESCRIPTION
This interface is made obsolete by regex(3). The re_comp() function compiles a string into an internal form suitable for pattern matching. The re_exec() function checks the argument string against the last string passed to re_comp(). The re_comp() function returns 0 if the string s was compiled successfully; otherwise a string containing an error message is returned. If re_comp() is passed 0 or a null string, it returns without changing the currently compiled regular expression. The re_exec() function returns 1 if the string s matches the last compiled regular expression, 0 if the string s failed to match the last compiled regular expression, and -1 if the compiled regular expression was invalid (indicating an internal error). The strings passed to both re_comp() and re_exec() may have trailing or embedded newline characters; they are terminated by NULs. The regu- lar expressions recognized are described in the manual entry for ed(1), given the above difference. DIAGNOSTICS
The re_exec() function returns -1 for an internal error. The re_comp() function returns ``no previous regular expression'' or one of the strings generated by regerror(3). SEE ALSO
ed(1), egrep(1), ex(1), fgrep(1), grep(1), regex(3) HISTORY
The re_comp() and re_exec() functions appeared in 4.0BSD. BSD
June 4, 1993 BSD
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