Anchors enable you to fix a regular expression to either the beginning or end of a line. They also enable you to create regular expressions that occur either within a word or at the beginning or end of a word.
Examples in previous topics in this section demonstrate how to find chapter headings. Any occurrence of the string "Chapter" followed by a space and a number could be an actual chapter heading, or it could also be a cross-reference to another chapter. Because chapter headings appear at the start of a line, it might be useful to devise a way to find only the headings and not find the cross-references. Anchors provide that capability.
The following table contains the list of regular expression anchors and their meanings:
Matches the position at the beginning of the input string. If the m (multiline search) character is included with the flags, ^ also matches the position following \n or \r.
Matches the position at the end of the input string. If the m (multiline search) character is included with the flags, $ also matches the position preceding \n or \r.
Matches a word boundary, that is, the position between a word and a space.
Matches a nonword boundary.
You cannot use a quantifier with an anchor. Since you cannot have more than one position immediately before or after a newline or word boundary, expressions such as ^* are not permitted.
To match text at the beginning of a line of text, use the ^ character at the beginning of the regular expression. Do not confuse this use of the ^ with the use within a bracket expression.
To match text at the end of a line of text, use the $ character at the end of the regular expression.
To use anchors when searching for chapter headings, the following regular expression matches a chapter heading with up to two following digits that occurs at the beginning of a line:
Not only does a true chapter heading occur at the beginning of a line, it is also the only text on the line, so it also must be at the end of a line as well. The following expression ensures that the match specified only matches chapters and not cross-references. It does so by creating a regular expression that matches only at the beginning and end of a line of text.
The position of the \b operator is critical. If it is positioned at the beginning of a string to be matched, it looks for the match at the beginning of the word; if it is positioned at the end of the string, it looks for the match at the end of the word. For example, the following expression matches "ter" in the word "Chapter" because it appears before a word boundary:
The following expression matches "apt" as it occurs in "Chapter", but not as it occurs in "aptitude":
The string "apt" occurs on a nonword boundary in the word "Chapter" but on a word boundary in the word "aptitude". For the \B nonword boundary operator, position is not important because the match is not relative to the beginning or end of a word.