Regular Expressions

A refresher concerning the syntax and rules of constructing regular expressions.

See also Configuring an incremental index of a staged website.

Syntax of regular expressions

Rules about regular expressions

• An ordinary character—not one of the special characters described below—is a one-character regular expression that matches itself.

• A backslash () followed by any special character is a one-character regular expression that matches the special character itself. Special characters include the following:

  • . (period), * (asterisk), ? (question mark), + (plus sign), [ (left square bracket), | (vertical pipe), and \ (backslash) are always special characters, except when they appear within square brackets.

  • ^ (caret or circumflex) is special at the beginning of a regular expression, or when it immediately follows the left of a pair of square brackets.

  • $ (dollar sign) is special at the end of a regular expression.

  • . (period) is a one-character regular expression that matches any character, including supplementary code set characters with the exception of new-line.

  • A non-empty string of characters enclosed in [ ] (left and right square brackets) is a one-character regular expression that matches one character, including supplementary code set characters, in that string.

    If, however, the first character of the string is a ^ (circumflex), the one-character regular expression matches any character, including supplementary code set characters, with the exception of new-line and the remaining characters in the string.

    The ^ has this special meaning only if it occurs first in the string. You can use - (minus sign) to indicate a range of consecutive characters, including supplementary code set characters. For example, [0-9] is equivalent to [0123456789].

    Characters specifying the range must be from the same code set. When the characters are from different code sets, one of the characters specifying the range is matched. The - loses this special meaning if it occurs first (after an initial ^, if any) or last in the string. The ] (right square bracket) does not terminate such a string when it is the first character within it, after an initial ^, if any. For example, []a-f] matches either a ] (right square bracket) or one of the ASCII letters a through f inclusive. The four characters listed as special characters above stand for themselves within such a string of characters.

Rules for constructing regular expressions from one-character regular expressions

You can use the following rules to construct regular expressions from one-character regular expressions:

• A one-character regular expression is a regular expression that matches whatever the one-character regular expression matches.
• A one-character regular expression followed by a * (asterisk) is a regular expression that matches zero or more occurrences of the one-character regular expression, which may be a supplementary code set character. If there is any choice, the longest leftmost string that permits a match is chosen.
• A one-character regular expression followed by a ? (question mark) is a regular expression that matches zero or one occurrences of the one-character regular expression, which may be a supplementary code set character. If there is any choice, the longest leftmost string that permits a match is chosen.
• A one-character regular expression followed by a + (plus sign) is a regular expression that matches one or more occurrences of the one-character regular expression, which may be a supplementary code set character. If there is any choice, the longest leftmost string that permits a match is chosen.
• A one-character regular expression followed by {m}, {m,}, or {m,n} is a regular expression that matches a range of occurrences of the one-character regular expression. The values of m and n must be non-negative integers less than 256; {m} matches exactly m occurrences; {m,} matches at least m occurrences; {m,n} matches any number of occurrences between m and n inclusive. Whenever a choice exists, the regular expression matches as many occurrences as possible.
• The concatenation of regular expressions is a regular expression that matches the concatenation of the strings matched by each component of the regular expression.
• A regular expression enclosed between the character sequences ( and ) is a regular expression that matches whatever the unadorned regular expression matches.
• A regular expression followed by a | (vertical pipe) followed by a regular expression is a regular expression that matches either the first regular expression (before the vertical pipe) or the second regular expression (after the vertical pipe).

You can also constrain a regular expression to match only an initial segment or final segment of a line, or both.

• A ^ (circumflex) at the beginning of a regular expression constrains that regular expression to match an initial segment of a line.
• A $ (dollar sign) at the end of an entire regular expression constrains that regular expression to match a final segment of a line.
• The construction ^regular expression$ constrains the regular expression to match the entire line.

There are some predefined character class names that you can use in place of complex bracketed regular expressions. For example, a digit can be represented by the one-character regular expression [0-9] or by the character class one-character regular expression [[:digit:]].

The predefined character classes and their meanings are the following:

Two special character class names match the null space at the start and the end of a word. In other words, they do not match an actual character. A word is considered to be any sequence of alphabetic characters, digits, or underscores (_).