In regular expressions, most characters just match themselves. The exceptions are the "metacharacters" that have special meaning.

In regexpr, the special characters are: "" (backslash), "." (dot), "*" (asterisk), "[" (bracket), "^" (caret, special only at the beginnig of a string), "$" (dollar sign, special only at the end of a string). A character terminating a pattern string is also special for this string.

The backslash, "" is used as an "escape" character, i.e., to quote a subsequent special character.
Thus, "" searches for a backslash, "." searches for a dot, "*" searches for the asterisk, "[" searches for the bracket, "^" searches for the caret even at the begining of the string, "$" searches for the dollar sign even at the end of the string.

Backslash followed by a regular (non-special) character may gain a special meaning. Thus, the symbols < and > match an empty string at the beginning and the end of a word, respectively. The symbol  matches the empty string at the edge of a word, and B matches the empty string provided it's not at the edge of a word.

The dot, ".", matches any single character. [The dir command uses "?" in this place.] Thus, "m.a" matches "mpa" and "mea" but not "ma" or "mppa".

Any string is matched by ".*" (dot and asterisk). [The dir command uses "*" instead.] In general, any pattern followed by "*" matches zero or more occurences of this pattern. Thus, "m*" matches zero or more occurances of "m". To search for one or more "m", I could use "mm*".

The * is a repetition operator. Other repetition operators are used less often--here is the full list:
* the proceding item is to be matched zero or more times;
+ the preceding item is to be matched one or more times);
? the preceding item is optional and matched at most once);
{n} the preceding item is to be matched exactly n times;
{n,} the preceding item is to be matched n or more times);
{n,m} the preceding item is to be matched at least n times, but not more than m times.

The caret, "^", means "the beginning of the line". So "^a" means "find a line starting with an "a".

The dollar sign, "$", means "the end of the line". So "a$" means "find a line ending with an "a".

Example. This command searches the file myfile for lines starting with an "s" and ending with an "n", and prints them to the standard output (screen):

cat myfile | grep '^s.*n$'

Any character terminating the pattern string is special, precede it with a backslash if you want to use it within this string.

The bracket, "[" introduces a set. Thus [abD] means: either a or b or D. [a-zA-C] means any character from a to z or from A to C.

Attention with some characters inside sets. Within a set, the only special characters are "[", "]", "-", and "^", and the combinations "[:", "[=", and "[.". The backslash is not special within a set.

Some useful categories of characters are: [:upper:] =upper-case letters, [:lower:] =lower-case letters, [:alpha:] =letters meaning upper+lower, [:digit:] =0 to 9, [:alnum:] = alpha and digits, [:space:] =whitespace meaning ++ and similar, [:graph:] =graphically printable characters except space, [:print:] =printable characters including space, [:punct:] =punctuation characters meaning graphical characters minus alpha and digits, [:cntrl:] =control characters meaning non-printable characters.

Example. This command prints lines containing a capital letter followed by a digit:

dir -l | grep '[[:upper:]][[:digit:]]'