Regular Expressions are used in the search (and replace) operations. The following notes are applicable when magic(2m) mode is enabled.
A "regular expression" (or "regex", or "pattern") is a text string that describes some (mathematical) set of strings. A regex R "matches" a string S if S is in the set of strings described by R.
Some regular expressions match only one string, i.e., the set they describe has only one member. For example, the regular expression 'foo' matches the string 'foo' and no others. Other regular expressions match more than one string, i.e., the set they describe has more than one member. For example, the regular expression 'f*' matches the set of strings made up of any number (including zero) of 'f's. As you can see, some characters in regular expressions match themselves (such as 'f') and some don't (such as '*'); the ones that do not match themselves instead let you specify patterns that describe many different strings.
Regular expressions have a syntax in which a few characters are special constructs and the rest are "ordinary". An ordinary character is a simple regular expression which matches that same character and nothing else. The special characters are '$', '^', '.', '*', '+', '?', '[', ']' and '\'. Any other character appearing in a regular expression is ordinary, unless a '\' precedes it.
For example, 'f' is not a special character, so it is ordinary, and therefore 'f' is a regular expression that matches the string 'f' and no other string. (It does not match the string 'ff'.) Likewise, 'o' is a regular expression that matches only 'o'. (When case distinctions are being ignored, these regexs also match 'F' and 'O', but we consider this a generalization of "the same string", rather than an exception.)
Any two regular expressions A and B can be concatenated. The result is a regular expression which matches a string if A matches some amount of the beginning of that string and B matches the rest of the string.
As a simple example, we can concatenate the regular expressions 'f' and 'o' to get the regular expression 'fo', which matches only the string 'fo'. Still trivial. To do something nontrivial, you need to use one of the special characters. Here is a list of them.
'*' always applies to the smallest possible preceding expression. Thus, 'fo*' has a repeating 'o', not a repeating 'fo'. It matches 'f', 'fo', 'foo', and so on.
The matcher processes a '*' construct by matching, immediately, as many repetitions as can be found. Then it continues with the rest of the pattern. If that fails, backtracking occurs, discarding some of the matches of the '*'-modified construct in case that makes it possible to match the rest of the pattern. For example, in matching 'ca*ar' against the string 'caaar', the 'a*' first tries to match all three 'a's; but the rest of the pattern is 'ar' and there is only 'r' left to match, so this try fails. The next alternative is for 'a*' to match only two 'a's. With this choice, the rest of the regex matches successfully.
+ (plus) is a postfix operator, similar to '*' except that it must match the preceding expression at least once. So, for example, 'ca+r' matches the strings 'car' and 'caaaar' but not the string 'cr', whereas 'ca*r' matches all three strings.
'?' (question mark)
[ ... ]
Thus, '[ad]' matches either one 'a' or one 'd', and '[ad]*' matches any string composed of just 'a's and 'd's (including the empty string), from which it follows that 'c[ad]*r' matches 'cr', 'car', 'cdr', 'caddaar', etc.
You can also include character ranges in a character set, by writing the starting and ending characters with a '-' between them. Thus, '[a-z]' matches any lower-case ASCII letter. Ranges may be intermixed freely with individual characters, as in '[a-z$%.]', which matches any lower-case ASCII letter or '$', '%' or period.
The word and digit special characters (i.e. \w, \l, etc.) are recognized within the character set.
To include a ']' in a character set, you must make it the first character. For example, 'a]' matches ']' or 'a'. To include a '-', write '-' as the first or last character of the set, or put it after a range. Thus, '-]' matches both ']' and '-'.
To include '^' in a set, put it anywhere but at the beginning of the set.
When you use a range in case-insensitive search, you should write both ends of the range in upper case, or both in lower case, or both should be non-letters. The behavior of a mixed-case range such as 'A-z' is somewhat ill-defined, and it may change in future Emacs versions.
[^ ... ]
'^' is not special in a character set unless it is the first character. The character following the '^' is treated as if it were first (in other words, '-' and ']' are not special there).
A complemented character set can match a newline, unless newline is mentioned as one of the characters not to match. This is in contrast to the handling of regexs in programs such as grep(1).
Because '\' quotes special characters, '\$' is a regular expression that matches only '$', and '\[' is a regular expression that matches only '[', and so on.
Note: for historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense. For example, '*foo' treats '*' as ordinary since there is no preceding expression on which the '*' can act. It is poor practice to depend on this behavior; it is better to quote the special character anyway, regardless of where it appears.
For the most part, '\' followed by any character matches only that character. However, there are several exceptions: two-character sequences starting with '\' that have special meanings. The second character in the sequence is always an ordinary character when used on its own. Here is a table of '\' constructs.
Thus, 'foo\|bar' matches either 'foo' or 'bar' but no other string.
'\|' applies to the largest possible surrounding expressions. Only a surrounding '\( ... \)' grouping can limit the grouping power of '\|'.
Full backtracking capability exists to handle multiple uses of '\|'.
\( ... \)
After the end of a '\( ... \)' construct, the matcher remembers the beginning and end of the text matched by that construct. Then, later on in the regular expression, you can use '\' followed by the digit D to mean "match the same text matched the Dth time by the '\( ... \)' construct."
The strings matching the first nine '\( ... \)' constructs appearing in a regular expression are assigned numbers 1 through 9 in the order that the open-parentheses appear in the regular expression. So you can use '\1' through '\9' to refer to the text matched by the corresponding '\( ... \)' constructs.
For example, '\(.*\)\1' matches any newline-free string that is composed of two identical halves. The '\(.*\)' matches the first half, which may be anything, but the '\1' that follows must match the same exact text.
If a particular '\( ... \)' construct matches more than once (which can easily happen if it is followed by '*'), only the last match is recorded.
NOTE: This currently only matches the start of the current line - it does not match the start of the buffer.
NOTE: This currently only matches the end of the current line - it does not match the end of the buffer.
The constructs that pertain to words and syntax are controlled by the setting of the syntax table.
A regular expression replacement, query-replace-string(2) command (with magic(2m) mode enabled), replaces exact matches for a single string or pattern. The replacement pattern may be a constant; it may also refer to all or part of what is matched by the regular expression search string.
M-x query-replace-string<RET> c[ad]+r <RET> \&-safe <RET>
replaces (for example) "cadr" with "cadr-safe" and "cddr" with "cddr-safe".
M-x query-replace-string<RET> \(c[ad]+r\)-safe <RET> \1 <RET>
performs the inverse transformation.
\0 is a special case, this represents the whole of the search pattern, it is equivalent to \&.
Searching may be either case sensitive or case insensitive, and is controlled by the exact(2m) mode. When exact mode is enabled (default) the then searches are case sensitive; disabled then case is ignored. The exact(2m) mode is set on a per-buffer basis.
The search engine searches for the longest string that matches a given pattern, the longest pattern is sometimes the pattern that is not actually required. For instance, consider searching for an HTML bracket set. The simplest search is:-
M-x search-forward "<.*>"
Unfortunately, this pattern is not specific enough, given an HTML line:-
<a href="www.jasspa.com">Jasspa Site</a>
Then the pattern matched is actually the whole line as the .* matches everything to the last >, this is the longest string. To rectify the pattern then we must be more specific, the correct search pattern to use in this instance is:-
M-x search-forward "<[^>]*>"
In this case we match any character excluding the closing character, this guarantees that we always find the shortest string match. A search of our HTML line locates two separate instances of the regular expression <a href="www.jasspa.com"> and </a>.
As of March 2005 then the existing Emacs search string syntax of "\s?" and "\S?" has been discarded in favor of Perl's simpler syntax of "\s" and "\S" for a whitespace character class. Added new character classes "\d" = digits, "\D" != digits "\h" = hexdigits "\H" != hexdigits "\l" = lowercase "\L" != lowercase "\u" = uppercase and "\U" != uppercase. The regular expression replacement string characters include "\c#" "\l#" and "\u#" to change the case of groups.
Copyright (c) 1998-2006 JASSPA
Last Modified: 2006/08/19
Generated On: 2006/10/07