reg_match() returns non-zero if the regular expression is matched in the argument string. For example, the condition, <?if (reg_match("^This.*", "This is an example string")> would be true since the "^This.*" expression says to match the word This at the beginning of the string and then match any characters afterwards.
reg_search() will scan the entire argument string for any matches to the given regular expression. If a match is found, it will return the portion of the string starting at where the patch occurred. If no match is found a zero-length string is returned.
reg_replace() scans the entire argument string and replaces any portions of the string matched by the given expression with the replacement string. For examples, in the string, "This is an example string" we could very easily replace every space with a dash with the command: reg_replace(" ","-","This is an example string").