Specifications

The second use is to validate customer email addresses in our application by encoding the stan-

dardized format of an email address in a regular expression. The format includes some

alphanumeric or punctuation characters, followed by an @ symbol, followed by a string of

alphanumeric and hyphen characters, followed by a dot, followed by more alphanumeric and

hyphen characters and possibly more dots, up until the end of the string, which encodes as fol-

lows:

^[a-zA-Z0-9_]+@[a-zA-Z0-9\-]+\.[a-zA-Z0-9\-\.]+$

The subexpression ^[a-zA-Z0-9_]+ means “start the string with at least one letter, number, or

underscore, or some combination of those.”

The @ symbol matches a literal @.

The subexpression [a-zA-Z0-9\-]+ matches the first part of the host name including alphanu-

meric characters and hyphens. Note that we’ve slashed out the hyphen because it’s a special

character inside square brackets.

The \. combination matches a literal ..

The subexpression [a-zA-Z0-9\-\.]+$ matches the rest of a domain name, including letters,

numbers, hyphens, and more dots if required, up until the end of the string.

A bit of analysis shows that you can produce invalid email addresses that will still match this

regular expression. It is almost impossible to catch them all, but this will improve the situation

a little.

Now that you have read about regular expressions, we’ll look at the PHP functions that use

them.

Finding Substrings with Regular Expressions

Finding substrings is the main application of the regular expressions we just developed. The

two functions available in PHP for matching regular expressions are ereg() and eregi().

The ereg() function has the following prototype:

int ereg(string pattern, string search, array [matches]);

This function searches the search string, looking for matches to the regular expression in

pattern. If matches are found for subexpressions of pattern, they will be stored in the array

matches, one subexpression per array element.

The eregi() function is identical except that it is not case sensitive.

Using PHP

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