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regex quick guide

Some people actually claim regular expressions (regex) is tricky. This little overview will prove them wrong... :-)

==============================================================================
			     Regex Guide
==============================================================================

 This is a small summary of most of the regex operators and how to use them.


        \        Quote the next metacharacter
        ^        Match the beginning of the line
        .        Match any character (except newline)
        $        Match the end of the line
        |        Alternation (or)
	[abc]    Match only a, b or c
	[^abc]   Match anything except a, b and c
 
        *        Match 0 or more times
        +        Match 1 or more times
        ?        Match 1 or 0 times
        {n}      Match exactly n times
        {n,}     Match at least n times
        {n,m}    Match at least n but not more than m times
 
        \w       Match a "word" character (alphanumeric plus "_")
        \W       Match a non-word character
        \s       Match a whitespace character
        \S       Match a non-whitespace character


 You compose regular expressions from operators. Most operators have more than
one representation as characters.

====================
1. Operator Overview
====================

  Match-self Operator                 Ordinary characters.
  Match-any-character Operator        .
  Concatenation Operator              Juxtaposition.
  Repetition Operators                *  +  ? {}
  Alternation Operator                |
  List Operators                      [...]  [^...]
  Grouping Operators                  (...)
  Back-reference Operator             \digit
  Anchoring Operators                 ^  $


The Match-self Operator (ORDINARY CHARACTER)
============================================

  This operator matches the character itself.  All ordinary characters
represent this operator. For example, `f' is always an ordinary character, so
the regular expression `f' matches only the string `f'.  In particular, it
does *not* match the string `ff'.

The Match-any-character Operator (`.')
======================================

  This operator matches any single printing or nonprinting character
except it won't match a newline.

  The `.' (period) character represents this operator.  For example,
`a.b' matches any three-character string beginning with `a' and ending
with `b'.

The Concatenation Operator
==========================

  This operator concatenates two regular expressions A and B.  No character
represents this operator; you simply put B after A.  The result is a regular
expression that will match a string if A matches its first part and B matches
the rest.  For example, `xy' (two match-self operators) matches `xy'.


Repetition Operators
====================

  Repetition operators repeat the preceding regular expression a specified
number of times.

  Match-zero-or-more  *
  Match-one-or-more   +
  Match-zero-or-one   ?
  Interval            {}


	The Match-zero-or-more Operator (`*')
	-------------------------------------

	This operator repeats the smallest possible preceding regular
	expression as many times as necessary (including zero) to match the
	pattern.  `*' represents this operator.  For example, `o*' matches any
	string made up of zero or more `o's.  Since this operator operates on
	the smallest preceding regular expression, `fo*' has a repeating `o',
	not a repeating `fo'.  So, `fo*' matches `f', `fo', `foo', and so on.

	  Since the match-zero-or-more operator is a suffix operator, it may
	be useless as such when no regular expression precedes it.  This is
	the case when it:

	   * is first in a regular expression, or

	   * follows a match-beginning-of-line, open-group, or alternation
	     operator.

	  The matcher processes a match-zero-or-more operator by first
	matching as many repetitions of the smallest preceding regular
	expression as it can.  Then it continues to match the rest of the
	pattern.

	  If it can't match the rest of the pattern, it backtracks (as many
	times as necessary), each time discarding one of the matches until it
	can either match the entire pattern or be certain that it cannot get a
	match.  For example, when matching `ca*ar' against `caaar', the
	matcher first matches all three `a's of the string with the `a*' of
	the regular expression.  However, it cannot then match the final `ar'
	of the regular expression against the final `r' of the string.  So it
	backtracks, discarding the match of the last `a' in the string.  It
	can then match the remaining `ar'.

	The Match-one-or-more Operator (`+')
	------------------------------------

	  This operator is similar to the match-zero-or-more operator except
	that it repeats the preceding regular expression at least once.

	  For example, `ca+r' matches, e.g., `car' and `caaaar', but not `cr'.

	The Match-zero-or-one Operator (`?')
	------------------------------------

	  This operator is similar to the match-zero-or-more operator except
	that it repeats the preceding regular expression once or not at all.

	  For example, `ca?r' matches both `car' and `cr', but nothing else.

	Interval Operators (`{' ... `}'
	-------------------------------

	`{COUNT}'
	     matches exactly COUNT occurrences of the preceding regular
	     expression.

	`{MIN,}'
	     matches MIN or more occurrences of the preceding regular
	     expression.

	`{MIN, MAX}'
	     matches at least MIN but no more than MAX occurrences of the
	     preceding regular expression.

	  The interval expression (but not necessarily the regular expression
	that contains it) is invalid if:

	   * MIN is greater than MAX, or

	   * any of COUNT, MIN, or MAX are outside the range zero to
	     `RE_DUP_MAX' (which symbol `regex.h' defines).


The Alternation Operator (`|')
==============================

  Alternatives match one of a choice of regular expressions: if you put the
character(s) representing the alternation operator between any two regular
expressions A and B, the result matches the union of the strings that A and B
match.  For example, supposing that `|' is the alternation operator, then
`foo|bar|quux' would match any of `foo', `bar' or `quux'.

  The alternation operator operates on the *largest* possible surrounding
regular expressions.  (Put another way, it has the lowest precedence of any
regular expression operator.) Thus, the only way you can delimit its arguments
is to use grouping.  For example, if `(' and `)' are the open and close-group
operators, then `fo(o|b)ar' would match either `fooar' or `fobar'.  (`foo|bar'
would match `foo' or `bar'.)

  The matcher usually tries all combinations of alternatives so as to match
the longest possible string.  For example, when matching
`(fooq|foo)*(qbarquux|bar)' against `fooqbarquux', it cannot take, say, the
first ("depth-first") combination it could match, since then it would be
content to match just `fooqbar'.

List Operators (`[' ... `]')
============================

  "Lists", also called "bracket expressions", are a set of one or more items.
An "item" is a character, a character class expression, or a range expression.
The syntax bits affect which kinds of items you can put in a list.  We explain
the last two items in subsections below.  Empty lists are invalid.

  A "matching list" matches a single character represented by one of the list
items.  You form a matching list by enclosing one or more items within an
"open-matching-list operator" (represented by `[') and a "close-list operator"
(represented by `]').

  For example, `[ab]' matches either `a' or `b'.  `[ad]*' matches the empty
string and any string composed of just `a's and `d's in any order.  Regex
considers invalid a regular expression with a `[' but no matching `]'.

  "Nonmatching lists" are similar to matching lists except that they match a
single character *not* represented by one of the list items.  You use an
"open-nonmatching-list operator" (represented by `[^'(1)) instead of an
open-matching-list operator to start a nonmatching list.

  For example, `[^ab]' matches any character except `a' or `b'.

  Most characters lose any special meaning inside a list.  The special
characters inside a list follow.

`]'
     ends the list if it's not the first list item.  So, if you want to
     make the `]' character a list item, you must put it first.

`[:'
     represents the open-character-class operator if what follows is a valid
     character class expression.

`:]'
     represents the close-character-class operator if what precedes it is an
     open-character-class operator followed by a valid character class name.

`-'
     represents the range operator if it's not first or last in a list or the
     ending point of a range.

All other characters are ordinary.  For example, `[.*]' matches `.' and
`*'.

  Character Class   [:class:]
  Range             start-end

  ---------- Footnotes ----------

  (1)  Regex therefore doesn't consider the `^' to be the first
character in the list.  If you put a `^' character first in (what you
think is) a matching list, you'll turn it into a nonmatching list.

	Character Class Operators (`[:' ... `:]')
	-----------------------------------------

	A "character class expression" matches one character from a given
	class.  You form a character class expression by putting a character
	class name between an "open-character-class operator" (represented by
	`[:') and a "close-character-class operator" (represented by `:]').
	The character class names and their meanings are:

	`alnum'
	     letters and digits

	`alpha'
	     letters

	`blank'
	     system-dependent; for GNU, a space or tab

	`cntrl'
	     control characters (in the ASCII encoding, code 0177 and codes
	     less than 040)

	`digit'
	     digits
	
	`graph'
	     same as `print' except omits space

	`lower'
	     lowercase letters

	`print'
	     printable characters (in the ASCII encoding, space tilde--codes
	     040 through 0176)

	`punct'
	     neither control nor alphanumeric characters

	`space'
	     space, carriage return, newline, vertical tab, and form feed

	`upper'
	     uppercase letters

	`xdigit'
	     hexadecimal digits: `0'-`9', `a'-`f', `A'-`F'

	These correspond to the definitions in the C library's `<ctype.h>'
	facility.  For example, `[:alpha:]' corresponds to the standard
	facility `isalpha'.  Regex recognizes character class expressions only
	inside of lists; so `[[:alpha:]]' matches any letter, but `[:alpha:]'
	outside of a bracket expression and not followed by a repetition
	operator matches just itself.

	The Range Operator (`-')
	------------------------

	  Regex recognizes "range expressions" inside a list. They represent
	those characters that fall between two elements in the current
	collating sequence.  You form a range expression by putting a "range
	operator" between two characters.(1) `-' represents the range
	operator.  For example, `a-f' within a list represents all the
	characters from `a' through `f' inclusively.

	  Since `-' represents the range operator, if you want to make a `-'
	character itself a list item, you must do one of the following:

	   * Put the `-' either first or last in the list.

	   * Include a range whose starting point collates strictly lower than
	     `-' and whose ending point collates equal or higher.  Unless a
	     range is the first item in a list, a `-' can't be its starting
	     point, but *can* be its ending point.  That is because Regex
	     considers `-' to be the range operator unless it is preceded by
	     another `-'.  For example, in the ASCII encoding, `)', `*', `+',
	     `,', `-', `.', and `/' are contiguous characters in the collating
	     sequence.  You might think that `[)-+--/]' has two ranges: `)-+'
	     and `--/'.  Rather, it has the ranges `)-+' and `+--', plus the
	     character `/', so it matches, e.g., `,', not `.'.

	   * Put a range whose starting point is `-' first in the list.

	  For example, `[-a-z]' matches a lowercase letter or a hyphen (in
	English, in ASCII).

	  ---------- Footnotes ----------

	  (1) You can't use a character class for the starting or ending point
	of a range, since a character class is not a single character.


Grouping Operators (`(' ... `)')
================================

  A "group", also known as a "subexpression", consists of an "open-group
operator", any number of other operators, and a "close-group operator".  Regex
treats this sequence as a unit, just as mathematics and programming languages
treat a parenthesized expression as a unit.

  Therefore, using "groups", you can:

   * delimit the argument(s) to an alternation operator or a repetition
     operator

   * keep track of the indices of the substring that matched a given
     group. for a precise explanation.  This lets you:

        * use the back-reference operator

        * use registers


The Back-reference Operator ("\"DIGIT)
======================================

   A back reference matches a specified preceding group.  The back reference
operator is represented by `\DIGIT' anywhere after the end of a regular
expression's DIGIT-th group.

  DIGIT must be between `1' and `9'.  The matcher assigns numbers 1 through 9
to the first nine groups it encounters.  By using one of `\1' through `\9'
after the corresponding group's close-group operator, you can match a
substring identical to the one that the group does.

  Back references match according to the following (in all examples below, `('
represents the open-group, `)' the close-group, `{' the open-interval and `}'
the close-interval operator):

   * If the group matches a substring, the back reference matches an
     identical substring.  For example, `(a)\1' matches `aa' and
     `(bana)na\1bo\1' matches `bananabanabobana'.  Likewise, `(.*)\1' matches
     any newline-free string that is composed of two identical halves; the
     `(.*)' matches the first half and the `\1' matches the second half.

   * If the group matches more than once (as it might if followed by,
     e.g., a repetition operator), then the back reference matches the
     substring the group *last* matched.  For example, `((a*)b)*\1\2' matches
     `aabababa'; first group 1 (the outer one) matches `aab' and group 2 (the
     inner one) matches `aa'.  Then group 1 matches `ab' and group 2 matches
     `a'.  So, `\1' matches `ab' and `\2' matches `a'.

   * If the group doesn't participate in a match, i.e., it is part of an
     alternative not taken or a repetition operator allows zero repetitions of
     it, then the back reference makes the whole match fail.  For example,
     `(one()|two())-and-(three\2|four\3)' matches `one-and-three' and
     `two-and-four', but not `one-and-four' or `two-and-three'.  For example,
     if the pattern matches `one-and-', then its group 2 matches the empty
     string and its group 3 doesn't participate in the match.  So, if it then
     matches `four', then when it tries to back reference group 3--which it
     will attempt to do because `\3' follows the `four'--the match will fail
     because group 3 didn't participate in the match.

  You can use a back reference as an argument to a repetition operator.  For
example, `(a(b))\2*' matches `a' followed by two or more `b's.  Similarly,
`(a(b))\2{3}' matches `abbbb'.

  If there is no preceding DIGIT-th subexpression, the regular expression is
invalid.

Anchoring Operators
===================

  These operators can constrain a pattern to match only at the
beginning or end of the entire string or at the beginning or end of a
line.

  Match-beginning-of-line   ^
  Match-end-of-line         $


	The Match-beginning-of-line Operator (`^')
	------------------------------------------

	  This operator can match the empty string either at the beginning of
	the string or after a newline character.  Thus, it is said to "anchor"
	the pattern to the beginning of a line.

	  In the cases following, `^' represents this operator.  (Otherwise,
	`^' is ordinary.)

	   * It (the `^') is first in the pattern, as in `^foo'.

	   * It follows an open-group or alternation operator, as in `a\(^b\)'
	     and `a\|^b'.

	The Match-end-of-line Operator (`$')
	------------------------------------

	  This operator can match the empty string either at the end of the
	string or before a newline character in the string.  Thus, it is said
	to "anchor" the pattern to the end of a line.

	  It is always represented by `$'.  For example, `foo$' usually
	matches, e.g., `foo' and, e.g., the first three characters of
	`foo\nbar'.

============================
2. Word and Buffer Operators
============================

Word Operators

  Match-word-boundary         \b
	  This operator (represented by `\b') matches the empty string at
	either the beginning or the end of a word.  For example, `\brat\b'
	matches the separate word `rat'.

  Match-within-word           \B
	  This operator (represented by `\B') matches the empty string within
	a word. For example, `c\Brat\Be' matches `crate', but `dirty \Brat'
	doesn't match `dirty rat'.
	
  Match-beginning-of-word     \<
	  This operator (represented by `\<') matches the empty string at the
	beginning of a word.

  Match-end-of-word           \>
	This operator (represented by `\>') matches the empty string at the
	end of a word.

  Match-word-constituent      \w
	This operator (represented by `\w') matches any word-constituent
	character.

  Match-non-word-constituent  \W
	This operator (represented by `\W') matches any character that is not
	word-constituent.

Buffer Operators

  ("buffers" in mail2sms are the whole input strings that you can match.)

  Match-beginning-of-buffer   \`
	This operator (represented by `\`') matches the empty string at the
	beginning of the buffer.

  Match-end-of-buffer         \'
	This operator (represented by `\'') matches the empty string at the
	end of the buffer.


Modified April 19, 2011