dreg.txt

emboss的linux版本的源代码

   but not underscore.

   All non-alphameric characters other than \, -, ^ (at the start) and
   the terminating ] are non-special in character classes, but it does no
   harm if they are escaped.

    VERTICAL BAR

   Vertical bar characters are used to separate alternative patterns. For
   example, the pattern

   gilbert|sullivan

   matches either "gilbert" or "sullivan". Any number of alternatives may
   appear, and an empty alternative is permitted (matching the empty
   string). The matching process tries each alternative in turn, from
   left to right, and the first one that succeeds is used. If the
   alternatives are within a subpattern (defined below), "succeeds" means
   matching the rest of the main pattern as well as the alternative in
   the subpattern.

    INTERNAL OPTION SETTING

   The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
   PCRE_EXTENDED options can be changed from within the pattern by a
   sequence of Perl option letters enclosed between "(?" and ")". The
   option letters are

       i  for PCRE_CASELESS
       m  for PCRE_MULTILINE
       s  for PCRE_DOTALL
       x  for PCRE_EXTENDED

   For example, (?im) sets caseless, multiline matching. It is also
   possible to unset these options by preceding the letter with a hyphen,
   and a combined setting and unsetting such as (?im-sx), which sets
   PCRE_CASELESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and
   PCRE_EXTENDED, is also permitted. If a letter appears both before and
   after the hyphen, the option is unset.

   When an option change occurs at top level (that is, not inside
   subpattern parentheses), the change applies to the remainder of the
   pattern that follows. If the change is placed right at the start of a
   pattern, PCRE extracts it into the global options (and it will
   therefore show up in data extracted by the pcre_fullinfo() function).

   An option change within a subpattern affects only that part of the
   current pattern that follows it, so

   (a(?i)b)c

   matches abc and aBc and no other strings (assuming PCRE_CASELESS is
   not used). By this means, options can be made to have different
   settings in different parts of the pattern. Any changes made in one
   alternative do carry on into subsequent branches within the same
   subpattern. For example,

   (a(?i)b|c)

   matches "ab", "aB", "c", and "C", even though when matching "C" the
   first branch is abandoned before the option setting. This is because
   the effects of option settings happen at compile time. There would be
   some very weird behaviour otherwise.

   The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed
   in the same way as the Perl-compatible options by using the characters
   U and X respectively. The (?X) flag setting is special in that it must
   always occur earlier in the pattern than any of the additional
   features it turns on, even when it is at top level. It is best put at
   the start.

    SUBPATTERNS

   Subpatterns are delimited by parentheses (round brackets), which can
   be nested. Marking part of a pattern as a subpattern does two things:

   1. It localizes a set of alternatives. For example, the pattern

   cat(aract|erpillar|)

   matches one of the words "cat", "cataract", or "caterpillar". Without
   the parentheses, it would match "cataract", "erpillar" or the empty
   string.

   2. It sets up the subpattern as a capturing subpattern (as defined
   above). When the whole pattern matches, that portion of the subject
   string that matched the subpattern is passed back to the caller via
   the ovector argument of pcre_exec(). Opening parentheses are counted
   from left to right (starting from 1) to obtain the numbers of the
   capturing subpatterns.

   For example, if the string "the red king" is matched against the
   pattern

   the ((red|white) (king|queen))

   the captured substrings are "red king", "red", and "king", and are
   numbered 1, 2, and 3, respectively.

   The fact that plain parentheses fulfil two functions is not always
   helpful. There are often times when a grouping subpattern is required
   without a capturing requirement. If an opening parenthesis is followed
   by a question mark and a colon, the subpattern does not do any
   capturing, and is not counted when computing the number of any
   subsequent capturing subpatterns. For example, if the string "the
   white queen" is matched against the pattern

   the ((?:red|white) (king|queen))

   the captured substrings are "white queen" and "queen", and are
   numbered 1 and 2. The maximum number of capturing subpatterns is
   65535, and the maximum depth of nesting of all subpatterns, both
   capturing and non-capturing, is 200.

   As a convenient shorthand, if any option settings are required at the
   start of a non-capturing subpattern, the option letters may appear
   between the "?" and the ":". Thus the two patterns

       (?i:saturday|sunday)
       (?:(?i)saturday|sunday)

   match exactly the same set of strings. Because alternative branches
   are tried from left to right, and options are not reset until the end
   of the subpattern is reached, an option setting in one branch does
   affect subsequent branches, so the above patterns match "SUNDAY" as
   well as "Saturday".

    REPETITION

   Repetition is specified by quantifiers, which can follow any of the
   following items:

       a literal data character
       the . meta-character
       the \C escape sequence
       escapes such as \d that match single characters
       a character class
       a back reference (see next section)
       a parenthesized subpattern (unless it is an assertion)

   The general repetition quantifier specifies a minimum and maximum
   number of permitted matches, by giving the two numbers in curly
   brackets (braces), separated by a comma. The numbers must be less than
   65536, and the first must be less than or equal to the second. For
   example:

   z{2,4}

   matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
   special character. If the second number is omitted, but the comma is
   present, there is no upper limit; if the second number and the comma
   are both omitted, the quantifier specifies an exact number of required
   matches. Thus

   [aeiou]{3,}

   matches at least 3 successive vowels, but may match many more, while

   \d{8}

   matches exactly 8 digits. An opening curly bracket that appears in a
   position where a quantifier is not allowed, or one that does not match
   the syntax of a quantifier, is taken as a literal character. For
   example, {,6} is not a quantifier, but a literal string of four
   characters.

   The quantifier {0} is permitted, causing the expression to behave as
   if the previous item and the quantifier were not present.

   For convenience (and historical compatibility) the three most common
   quantifiers have single-character abbreviations:

       *    is equivalent to {0,}
       +    is equivalent to {1,}
       ?    is equivalent to {0,1}

   It is possible to construct infinite loops by following a subpattern
   that can match no characters with a quantifier that has no upper
   limit, for example:

   (a?)*

   Earlier versions of Perl and PCRE used to give an error at compile
   time for such patterns. However, because there are cases where this
   can be useful, such patterns are now accepted, but if any repetition
   of the subpattern does in fact match no characters, the loop is
   forcibly broken.

   By default, the quantifiers are "greedy", that is, they match as much
   as possible (up to the maximum number of permitted times), without
   causing the rest of the pattern to fail. The classic example of where
   this gives problems is in trying to match comments in C programs.
   These appear between the sequences /* and */ and within the sequence,
   individual * and / characters may appear. An attempt to match C
   comments by applying the pattern

   /\*.*\*/

   to the string

       /* first command */  not comment  /* second comment */

   fails, because it matches the entire string owing to the greediness of
   the .* item.

   However, if a quantifier is followed by a question mark, it ceases to
   be greedy, and instead matches the minimum number of times possible,
   so the pattern

   /\*.*?\*/

   does the right thing with the C comments. The meaning of the various
   quantifiers is not otherwise changed, just the preferred number of
   matches. Do not confuse this use of question mark with its use as a
   quantifier in its own right. Because it has two uses, it can sometimes
   appear doubled, as in

   \d??\d

   which matches one digit by preference, but can match two if that is
   the only way the rest of the pattern matches.

   If the PCRE_UNGREEDY option is set (an option which is not available
   in Perl), the quantifiers are not greedy by default, but individual
   ones can be made greedy by following them with a question mark. In
   other words, it inverts the default behaviour.

   When a parenthesized subpattern is quantified with a minimum repeat
   count that is greater than 1 or with a limited maximum, more store is
   required for the compiled pattern, in proportion to the size of the
   minimum or maximum. If a pattern starts with .* or .{0,} and the
   PCRE_DOTALL option (equivalent to Perl's /s) is set, thus allowing the
   . to match newlines, the pattern is implicitly anchored, because
   whatever follows will be tried against every character position in the
   subject string, so there is no point in retrying the overall match at
   any position after the first. PCRE normally treats such a pattern as
   though it were preceded by \A.

   In cases where it is known that the subject string contains no
   newlines, it is worth setting PCRE_DOTALL in order to obtain this
   optimization, or alternatively using ^ to indicate anchoring
   explicitly.

   However, there is one situation where the optimization cannot be used.
   When .* is inside capturing parentheses that are the subject of a
   backreference elsewhere in the pattern, a match at the start may fail,
   and a later one succeed. Consider, for example:

   (.*)abc\1

   If the subject is "xyz123abc123" the match point is the fourth
   character. For this reason, such a pattern is not implicitly anchored.

   When a capturing subpattern is repeated, the value captured is the
   substring that matched the final iteration. For example, after

   (tweedle[dume]{3}\s*)+

   has matched "tweedledum tweedledee" the value of the captured
   substring is "tweedledee". However, if there are nested capturing
   subpatterns, the corresponding captured values may have been set in
   previous iterations. For example, after

       /(a|(b))+/

  PCRE PERFORMANCE

   Certain items that may appear in regular expression patterns are more
   efficient than others. It is more efficient to use a character class
   like [aeiou] than a set of alternatives such as (a|e|i|o|u). In
   general, the simplest construction that provides the required
   behaviour is usually the most efficient. Jeffrey Friedl's book
   contains a lot of discussion about optimizing regular expressions for
   efficient performance.

   When a pattern begins with .* not in parentheses, or in parentheses
   that are not the subject of a backreference, and the PCRE_DOTALL
   option is set, the pattern is implicitly anchored by PCRE, since it
   can match only at the start of a subject string. However, if
   PCRE_DOTALL is not set, PCRE cannot make this optimization, because
   the . meta-character does not then match a newline, and if the subject
   string contains newlines, the pattern may match from the character
   immediately following one of them instead of from the very start. For
   example, the pattern

   .*second

   matches the subject "first\nand second" (where \n stands for a newline
   character), with the match starting at the seventh character. In order
   to do this, PCRE has to retry the match starting after every newline
   in the subject.

   If you are using such a pattern with subject strings that do not
   contain newlines, the best performance is obtained by setting
   PCRE_DOTALL, or starting the pattern with ^.* to indicate explicit
   anchoring. That saves PCRE from having to scan along the subject
   looking for a newline to restart at.

   Beware of patterns that contain nested indefinite repeats. These can
   take a long time to run when applied to a string that does not match.
   Consider the pattern fragment

   (a+)*

   This can match "aaaa" in 33 different ways, and this number increases
   very rapidly as the string gets longer. (The * repeat can match 0, 1,
   2, 3, or 4 times, and for each of those cases other than 0, the +
   repeats can match different numbers of times.) When the remainder of
   the pattern is such that the entire match is going to fail, PCRE has
   in principle to try every possible variation, and this can take an
   extremely long time. An optimization catches some of the more simple
   cases such as

   (a+)*b

   where a literal character follows. Before embarking on the standard
   matching procedure, PCRE checks that there is a "b" later in the
   subject string, and if there is not, it fails the match immediately.
   However, when there is no following literal this optimization cannot
   be used. You can see the difference by comparing the behaviour of

   (a+)*\d