User Guide

672 Chapter 28: Validating Data
Special characters
Because special characters are the operators in regular expressions, in order to represent a special
character as an ordinary one, you must escape it by preceding it with a backslash. For example,
use two backslash characters (\\) to represent a backslash character.
Single-character regular expressions
The following rules govern regular expressions that match a single character:
• Special characters are: + * ? . [ ^ $ ( ) { | \
• Any character that is not a special character or escaped by being preceded by a backslash (\)
matches itself.
• A backslash (\) followed by any special character matches the literal character itself; that is, the
backslash escapes the special character.
• A period (.) matches any character except newline.
• A set of characters enclosed in brackets ([]) is a one-character regular expression that matches
any of the characters in that set. For example, “[akm]” matches an a, k, or m. If you include ]
(closing square bracket) in square brackets, it must be the first character. Otherwise, it does not
work, even if you use \].
• A dash can indicate a range of characters. For example, [a-z] matches any lowercase letter.
• If the first character of a set of characters in brackets is the caret (^), the expression matches any
character except those in the set. It does not match the empty string. For example: “[^akm]”
matches any character except a, k, or m. The caret loses its special meaning if it is not the first
character of the set.
• You can make regular expressions case insensitive by substituting individual characters with
character sets; for example, “[Nn][Ii][Cc][Kk]” is a case-insensitive pattern for the name Nick
(or NICK, or nick, or even nIcK).
• You can use the following escape sequences to match specific characters or character classes:
Escape
seq
Matches Escape
seq
Meaning
[\b] Backspace. \s Any of the following white space
characters: space, tab, form feed, and
line feed.
\b A word boundary, such as a space. \S Any character except the white space
characters matched by \s.
\B A nonword boundary. \t Tab.
\cX The control character Ctrl-x. For
example, \cv matches Ctrl-v, the
usual control character for pasting
text.
\v Vertical tab.
\d A digit character [0-9]. \w An alphanumeric character or
underscore. The equivalent of
[A-Za-z0-9_].