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CHAPTER 9 ■ PERFORMING FORM VALIDATION WITH REGULAR EXPRESSIONS
Matching one or more characters is good for verifying that at least one character was entered. For
instance, if you want to verify that a user enters at least one character into a form input and that the
character is a valid word character, you can use this pattern to validate the input: /\w+/.
Finally, matching a specific range of characters is especially useful when matching numeric ranges.
For instance, you can use this pattern to ensure a value is between 0 and 99: /\b\d{1,2}\b/.
In your example file, you use this regex pattern to find any words consisting of exactly four letters:
/(\b\w{4}\b)/ (see Figure 9-7).
Figure 9-7. Matching only words that consist of exactly four letters
Detecting the Beginning or End of a String
Additionally, you can force the pattern to match from the beginning or end of the string (or both). If the
pattern starts with a caret (^), the regex will only match if the pattern starts with a matching character. If
it ends with a dollar sign ($), the regex will match only if the string ends with the preceding matching
character.
You can combine these different symbols to make sure an entire string matches a pattern. This is
useful when validating input because you can verify that the user only submitted valid information. For
instance, you can you can use this regex pattern to verify that a username contains only the letters A-Z,
the numbers 0-9, and the underscore character: /^\w+$/.
Using Alternation
In some cases, it’s desirable to use either one pattern or another. This is called alternation, and it’s
accomplished using a pipe character (|). This approach allows you to define two or more possibilities for
a match. For instance, you can use this pattern to match either three-, six-, or seven-letter words in
regex.php: /\b(\w{3}|\w{6,7})\b/ (see Figure 9-8).