STFI.
http://www.regular-expressions.info/php.html
RTFM.
http://us3.php.net/eregi
http://us3.php.net/manual/en/reference.pcre.pattern.syntax.php
And as a general introduction, it's helpful to know that a regular expression is basically of the form
#expression#pattern_modifiers
where the # characters can be just that or other characters like / or maybe others. why are they there? I don't know...i think the basic idea is to delimit them from pattern_modifiers.
patter_modifiers consists of single letter flags like i and s which tell the pattern matching subroutine, ereg, to ignore case (i) and match white space (s). more info on patter modifiers here:
http://us3.php.net/manual/en/reference.pcre.pattern.modifiers.php
here is an example php regular expression pattern creation:
$pattern = "#[a-z]*#is"; // this pattern zero or more letters between a and z without regard to case
The whole syntax of a pattern can be a tricky business and takes practice. This pattern will match a sequence of letters or digits and requires at least one letter or digit:
$pattern = "#[a-z0-9]+#is";
The ^ and $ characters match the beginning and end of your search string - UNLESS you also use a pattern modifier m which puts ereg in multiline mode. In that case they would refer to the beginning and end of a line. I don't know much about multiline mode. it's kind of confusing to me.
so this partern might be useful, it should assert that your search string has at least one character and contains only letters and numbers:
$pattern = "#^[a-z0-9]+$#is";
