guile/doc/ref/api-regex.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2009, 2010, 2012
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Regular Expressions
@section Regular Expressions
@tpindex Regular expressions

@cindex regular expressions
@cindex regex
@cindex emacs regexp

A @dfn{regular expression} (or @dfn{regexp}) is a pattern that
describes a whole class of strings.  A full description of regular
expressions and their syntax is beyond the scope of this manual.

If your system does not include a POSIX regular expression library,
and you have not linked Guile with a third-party regexp library such
as Rx, these functions will not be available.  You can tell whether
your Guile installation includes regular expression support by
checking whether @code{(provided? 'regex)} returns true.

The following regexp and string matching features are provided by the
@code{(ice-9 regex)} module.  Before using the described functions,
you should load this module by executing @code{(use-modules (ice-9
regex))}.

@menu
* Regexp Functions::            Functions that create and match regexps.
* Match Structures::            Finding what was matched by a regexp.
* Backslash Escapes::           Removing the special meaning of regexp
                                meta-characters.
@end menu


@node Regexp Functions
@subsection Regexp Functions

By default, Guile supports POSIX extended regular expressions.  That
means that the characters @samp{(}, @samp{)}, @samp{+} and @samp{?} are
special, and must be escaped if you wish to match the literal characters
and there is no support for ``non-greedy'' variants of @samp{*},
@samp{+} or @samp{?}.

This regular expression interface was modeled after that
implemented by SCSH, the Scheme Shell.  It is intended to be
upwardly compatible with SCSH regular expressions.

Zero bytes (@code{#\nul}) cannot be used in regex patterns or input
strings, since the underlying C functions treat that as the end of
string.  If there's a zero byte an error is thrown.

Internally, patterns and input strings are converted to the current
locale's encoding, and then passed to the C library's regular expression
routines (@pxref{Regular Expressions,,, libc, The GNU C Library
Reference Manual}).  The returned match structures always point to
characters in the strings, not to individual bytes, even in the case of
multi-byte encodings.

@deffn {Scheme Procedure} string-match pattern str [start]
Compile the string @var{pattern} into a regular expression and compare
it with @var{str}.  The optional numeric argument @var{start} specifies
the position of @var{str} at which to begin matching.

@code{string-match} returns a @dfn{match structure} which
describes what, if anything, was matched by the regular
expression.  @xref{Match Structures}.  If @var{str} does not match
@var{pattern} at all, @code{string-match} returns @code{#f}.
@end deffn

Two examples of a match follow.  In the first example, the pattern
matches the four digits in the match string.  In the second, the pattern
matches nothing.

@example
(string-match "[0-9][0-9][0-9][0-9]" "blah2002")
@result{} #("blah2002" (4 . 8))

(string-match "[A-Za-z]" "123456")
@result{} #f
@end example

Each time @code{string-match} is called, it must compile its
@var{pattern} argument into a regular expression structure.  This
operation is expensive, which makes @code{string-match} inefficient if
the same regular expression is used several times (for example, in a
loop).  For better performance, you can compile a regular expression in
advance and then match strings against the compiled regexp.

@deffn {Scheme Procedure} make-regexp pat flag@dots{}
@deffnx {C Function} scm_make_regexp (pat, flaglst)
Compile the regular expression described by @var{pat}, and
return the compiled regexp structure.  If @var{pat} does not
describe a legal regular expression, @code{make-regexp} throws
a @code{regular-expression-syntax} error.

The @var{flag} arguments change the behavior of the compiled
regular expression.  The following values may be supplied:

@defvar regexp/icase
Consider uppercase and lowercase letters to be the same when
matching.
@end defvar

@defvar regexp/newline
If a newline appears in the target string, then permit the
@samp{^} and @samp{$} operators to match immediately after or
immediately before the newline, respectively.  Also, the
@samp{.} and @samp{[^...]} operators will never match a newline
character.  The intent of this flag is to treat the target
string as a buffer containing many lines of text, and the
regular expression as a pattern that may match a single one of
those lines.
@end defvar

@defvar regexp/basic
Compile a basic (``obsolete'') regexp instead of the extended
(``modern'') regexps that are the default.  Basic regexps do
not consider @samp{|}, @samp{+} or @samp{?} to be special
characters, and require the @samp{@{...@}} and @samp{(...)}
metacharacters to be backslash-escaped (@pxref{Backslash
Escapes}).  There are several other differences between basic
and extended regular expressions, but these are the most
significant.
@end defvar

@defvar regexp/extended
Compile an extended regular expression rather than a basic
regexp.  This is the default behavior; this flag will not
usually be needed.  If a call to @code{make-regexp} includes
both @code{regexp/basic} and @code{regexp/extended} flags, the
one which comes last will override the earlier one.
@end defvar
@end deffn

@deffn {Scheme Procedure} regexp-exec rx str [start [flags]]
@deffnx {C Function} scm_regexp_exec (rx, str, start, flags)
Match the compiled regular expression @var{rx} against
@code{str}.  If the optional integer @var{start} argument is
provided, begin matching from that position in the string.
Return a match structure describing the results of the match,
or @code{#f} if no match could be found.

The @var{flags} argument changes the matching behavior.  The following
flag values may be supplied, use @code{logior} (@pxref{Bitwise
Operations}) to combine them,

@defvar regexp/notbol
Consider that the @var{start} offset into @var{str} is not the
beginning of a line and should not match operator @samp{^}.

If @var{rx} was created with the @code{regexp/newline} option above,
@samp{^} will still match after a newline in @var{str}.
@end defvar

@defvar regexp/noteol
Consider that the end of @var{str} is not the end of a line and should
not match operator @samp{$}.

If @var{rx} was created with the @code{regexp/newline} option above,
@samp{$} will still match before a newline in @var{str}.
@end defvar
@end deffn

@lisp
;; Regexp to match uppercase letters
(define r (make-regexp "[A-Z]*"))

;; Regexp to match letters, ignoring case
(define ri (make-regexp "[A-Z]*" regexp/icase))

;; Search for bob using regexp r
(match:substring (regexp-exec r "bob"))
@result{} ""                  ; no match

;; Search for bob using regexp ri
(match:substring (regexp-exec ri "Bob"))
@result{} "Bob"               ; matched case insensitive
@end lisp

@deffn {Scheme Procedure} regexp? obj
@deffnx {C Function} scm_regexp_p (obj)
Return @code{#t} if @var{obj} is a compiled regular expression,
or @code{#f} otherwise.
@end deffn

@sp 1
@deffn {Scheme Procedure} list-matches regexp str [flags]
Return a list of match structures which are the non-overlapping
matches of @var{regexp} in @var{str}.  @var{regexp} can be either a
pattern string or a compiled regexp.  The @var{flags} argument is as
per @code{regexp-exec} above.

@example
(map match:substring (list-matches "[a-z]+" "abc 42 def 78"))
@result{} ("abc" "def")
@end  example
@end deffn

@deffn {Scheme Procedure} fold-matches regexp str init proc [flags]
Apply @var{proc} to the non-overlapping matches of @var{regexp} in
@var{str}, to build a result.  @var{regexp} can be either a pattern
string or a compiled regexp.  The @var{flags} argument is as per
@code{regexp-exec} above.

@var{proc} is called as @code{(@var{proc} match prev)} where
@var{match} is a match structure and @var{prev} is the previous return
from @var{proc}.  For the first call @var{prev} is the given
@var{init} parameter.  @code{fold-matches} returns the final value
from @var{proc}.

For example to count matches,

@example
(fold-matches "[a-z][0-9]" "abc x1 def y2" 0
              (lambda (match count)
                (1+ count)))
@result{} 2
@end example
@end deffn

@sp 1
Regular expressions are commonly used to find patterns in one string
and replace them with the contents of another string.  The following
functions are convenient ways to do this.

@c begin (scm-doc-string "regex.scm" "regexp-substitute")
@deffn {Scheme Procedure} regexp-substitute port match item @dots{}
Write to @var{port} selected parts of the match structure @var{match}.
Or if @var{port} is @code{#f} then form a string from those parts and
return that.

Each @var{item} specifies a part to be written, and may be one of the
following,

@itemize @bullet
@item
A string.  String arguments are written out verbatim.

@item
An integer.  The submatch with that number is written
(@code{match:substring}).  Zero is the entire match.

@item
The symbol @samp{pre}.  The portion of the matched string preceding
the regexp match is written (@code{match:prefix}).

@item
The symbol @samp{post}.  The portion of the matched string following
the regexp match is written (@code{match:suffix}).
@end itemize

For example, changing a match and retaining the text before and after,

@example
(regexp-substitute #f (string-match "[0-9]+" "number 25 is good")
                   'pre "37" 'post)
@result{} "number 37 is good"
@end example

Or matching a @sc{yyyymmdd} format date such as @samp{20020828} and
re-ordering and hyphenating the fields.

@lisp
(define date-regex
   "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
(define s "Date 20020429 12am.")
(regexp-substitute #f (string-match date-regex s)
                   'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
@result{} "Date 04-29-2002 12am. (20020429)"
@end lisp
@end deffn


@c begin (scm-doc-string "regex.scm" "regexp-substitute")
@deffn {Scheme Procedure} regexp-substitute/global port regexp target item@dots{}
@cindex search and replace
Write to @var{port} selected parts of matches of @var{regexp} in
@var{target}.  If @var{port} is @code{#f} then form a string from
those parts and return that.  @var{regexp} can be a string or a
compiled regex.

This is similar to @code{regexp-substitute}, but allows global
substitutions on @var{target}.  Each @var{item} behaves as per
@code{regexp-substitute}, with the following differences,

@itemize @bullet
@item
A function.  Called as @code{(@var{item} match)} with the match
structure for the @var{regexp} match, it should return a string to be
written to @var{port}.

@item
The symbol @samp{post}.  This doesn't output anything, but instead
causes @code{regexp-substitute/global} to recurse on the unmatched
portion of @var{target}.

This @emph{must} be supplied to perform a global search and replace on
@var{target}; without it @code{regexp-substitute/global} returns after
a single match and output.
@end itemize

For example, to collapse runs of tabs and spaces to a single hyphen
each,

@example
(regexp-substitute/global #f "[ \t]+"  "this   is   the text"
                          'pre "-" 'post)
@result{} "this-is-the-text"
@end example

Or using a function to reverse the letters in each word,

@example
(regexp-substitute/global #f "[a-z]+"  "to do and not-do"
  'pre (lambda (m) (string-reverse (match:substring m))) 'post)
@result{} "ot od dna ton-od"
@end example

Without the @code{post} symbol, just one regexp match is made.  For
example the following is the date example from
@code{regexp-substitute} above, without the need for the separate
@code{string-match} call.

@lisp
(define date-regex 
   "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
(define s "Date 20020429 12am.")
(regexp-substitute/global #f date-regex s
                          'pre 2 "-" 3 "-" 1 'post " (" 0 ")")

@result{} "Date 04-29-2002 12am. (20020429)"
@end lisp
@end deffn


@node Match Structures
@subsection Match Structures

@cindex match structures

A @dfn{match structure} is the object returned by @code{string-match} and
@code{regexp-exec}.  It describes which portion of a string, if any,
matched the given regular expression.  Match structures include: a
reference to the string that was checked for matches; the starting and
ending positions of the regexp match; and, if the regexp included any
parenthesized subexpressions, the starting and ending positions of each
submatch.

In each of the regexp match functions described below, the @code{match}
argument must be a match structure returned by a previous call to
@code{string-match} or @code{regexp-exec}.  Most of these functions
return some information about the original target string that was
matched against a regular expression; we will call that string
@var{target} for easy reference.

@c begin (scm-doc-string "regex.scm" "regexp-match?")
@deffn {Scheme Procedure} regexp-match? obj
Return @code{#t} if @var{obj} is a match structure returned by a
previous call to @code{regexp-exec}, or @code{#f} otherwise.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:substring")
@deffn {Scheme Procedure} match:substring match [n]
Return the portion of @var{target} matched by subexpression number
@var{n}.  Submatch 0 (the default) represents the entire regexp match.
If the regular expression as a whole matched, but the subexpression
number @var{n} did not match, return @code{#f}.
@end deffn

@lisp
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:substring s)
@result{} "2002"

;; match starting at offset 6 in the string
(match:substring
  (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
@result{} "7654"
@end lisp

@c begin (scm-doc-string "regex.scm" "match:start")
@deffn {Scheme Procedure} match:start match [n]
Return the starting position of submatch number @var{n}.
@end deffn

In the following example, the result is 4, since the match starts at
character index 4:

@lisp
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:start s)
@result{} 4
@end lisp

@c begin (scm-doc-string "regex.scm" "match:end")
@deffn {Scheme Procedure} match:end match [n]
Return the ending position of submatch number @var{n}.
@end deffn

In the following example, the result is 8, since the match runs between
characters 4 and 8 (i.e.@: the ``2002'').

@lisp
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:end s)
@result{} 8
@end lisp

@c begin (scm-doc-string "regex.scm" "match:prefix")
@deffn {Scheme Procedure} match:prefix match
Return the unmatched portion of @var{target} preceding the regexp match.

@lisp
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:prefix s)
@result{} "blah"
@end lisp
@end deffn

@c begin (scm-doc-string "regex.scm" "match:suffix")
@deffn {Scheme Procedure} match:suffix match
Return the unmatched portion of @var{target} following the regexp match.
@end deffn

@lisp
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:suffix s)
@result{} "foo"
@end lisp

@c begin (scm-doc-string "regex.scm" "match:count")
@deffn {Scheme Procedure} match:count match
Return the number of parenthesized subexpressions from @var{match}.
Note that the entire regular expression match itself counts as a
subexpression, and failed submatches are included in the count.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:string")
@deffn {Scheme Procedure} match:string match
Return the original @var{target} string.
@end deffn

@lisp
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:string s)
@result{} "blah2002foo"
@end lisp


@node Backslash Escapes
@subsection Backslash Escapes

Sometimes you will want a regexp to match characters like @samp{*} or
@samp{$} exactly.  For example, to check whether a particular string
represents a menu entry from an Info node, it would be useful to match
it against a regexp like @samp{^* [^:]*::}.  However, this won't work;
because the asterisk is a metacharacter, it won't match the @samp{*} at
the beginning of the string.  In this case, we want to make the first
asterisk un-magic.

You can do this by preceding the metacharacter with a backslash
character @samp{\}.  (This is also called @dfn{quoting} the
metacharacter, and is known as a @dfn{backslash escape}.)  When Guile
sees a backslash in a regular expression, it considers the following
glyph to be an ordinary character, no matter what special meaning it
would ordinarily have.  Therefore, we can make the above example work by
changing the regexp to @samp{^\* [^:]*::}.  The @samp{\*} sequence tells
the regular expression engine to match only a single asterisk in the
target string.

Since the backslash is itself a metacharacter, you may force a regexp to
match a backslash in the target string by preceding the backslash with
itself.  For example, to find variable references in a @TeX{} program,
you might want to find occurrences of the string @samp{\let\} followed
by any number of alphabetic characters.  The regular expression
@samp{\\let\\[A-Za-z]*} would do this: the double backslashes in the
regexp each match a single backslash in the target string.

@c begin (scm-doc-string "regex.scm" "regexp-quote")
@deffn {Scheme Procedure} regexp-quote str
Quote each special character found in @var{str} with a backslash, and
return the resulting string.
@end deffn

@strong{Very important:} Using backslash escapes in Guile source code
(as in Emacs Lisp or C) can be tricky, because the backslash character
has special meaning for the Guile reader.  For example, if Guile
encounters the character sequence @samp{\n} in the middle of a string
while processing Scheme code, it replaces those characters with a
newline character.  Similarly, the character sequence @samp{\t} is
replaced by a horizontal tab.  Several of these @dfn{escape sequences}
are processed by the Guile reader before your code is executed.
Unrecognized escape sequences are ignored: if the characters @samp{\*}
appear in a string, they will be translated to the single character
@samp{*}.

This translation is obviously undesirable for regular expressions, since
we want to be able to include backslashes in a string in order to
escape regexp metacharacters.  Therefore, to make sure that a backslash
is preserved in a string in your Guile program, you must use @emph{two}
consecutive backslashes:

@lisp
(define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))
@end lisp

The string in this example is preprocessed by the Guile reader before
any code is executed.  The resulting argument to @code{make-regexp} is
the string @samp{^\* [^:]*}, which is what we really want.

This also means that in order to write a regular expression that matches
a single backslash character, the regular expression string in the
source code must include @emph{four} backslashes.  Each consecutive pair
of backslashes gets translated by the Guile reader to a single
backslash, and the resulting double-backslash is interpreted by the
regexp engine as matching a single backslash character.  Hence:

@lisp
(define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))
@end lisp

The reason for the unwieldiness of this syntax is historical.  Both
regular expression pattern matchers and Unix string processing systems
have traditionally used backslashes with the special meanings
described above.  The POSIX regular expression specification and ANSI C
standard both require these semantics.  Attempting to abandon either
convention would cause other kinds of compatibility problems, possibly
more severe ones.  Therefore, without extending the Scheme reader to
support strings with different quoting conventions (an ungainly and
confusing extension when implemented in other languages), we must adhere
to this cumbersome escape syntax.