* Text Comparison:: Comparing characters or strings.
* String Conversion:: Converting to and from characters and strings.
* Formatting Strings:: @code{format}: Emacs's analogue of @code{printf}.
+* Custom Format Strings:: Formatting custom @code{format} specifications.
* Case Conversion:: Case conversion functions.
* Case Tables:: Customizing case conversion.
@end menu
NaNs and can lose precision and type, and @samp{#x%x} and @samp{#o%o}
can mishandle negative integers. @xref{Input Functions}.
+The functions described in this section accept a fixed set of
+specification characters. The next section describes a function
+@code{format-spec} which can accept custom specification characters,
+such as @samp{%a} or @samp{%z}.
+
+@node Custom Format Strings
+@section Custom Format Strings
+@cindex custom format string
+@cindex custom @samp{%}-sequence in format
+
+Sometimes it is useful to allow users and Lisp programs alike to
+control how certain text is generated via custom format control
+strings. For example, a format string could control how to display
+someone's forename, surname, and email address. Using the function
+@code{format} described in the previous section, the format string
+could be something like @w{@code{"%s %s <%s>"}}. This approach
+quickly becomes impractical, however, as it can be unclear which
+specification character corresponds to which piece of information.
+
+A more convenient format string for such cases would be something like
+@w{@code{"%f %l <%e>"}}, where each specification character carries
+more semantic information and can easily be rearranged relative to
+other specification characters, making such format strings more easily
+customizable by the user.
+
+The function @code{format-spec} described in this section performs a
+similar function to @code{format}, except it operates on format
+control strings that use arbitrary specification characters.
+
+@defun format-spec template spec-alist &optional only-present
+This function returns a string produced from the format string
+@var{template} according to conversions specified in @var{spec-alist},
+which is an alist (@pxref{Association Lists}) of the form
+@w{@code{(@var{letter} . @var{replacement})}}. Each specification
+@code{%@var{letter}} in @var{template} will be replaced by
+@var{replacement} when formatting the resulting string.
+
+The characters in @var{template}, other than the format
+specifications, are copied directly into the output, including their
+text properties, if any. Any text properties of the format
+specifications are copied to their replacements.
+
+Using an alist to specify conversions gives rise to some useful
+properties:
+
+@itemize @bullet
+@item
+If @var{spec-alist} contains more unique @var{letter} keys than there
+are unique specification characters in @var{template}, the unused keys
+are simply ignored.
+@item
+If @var{spec-alist} contains more than one association with the same
+@var{letter}, the closest one to the start of the list is used.
+@item
+If @var{template} contains the same specification character more than
+once, then the same @var{replacement} found in @var{spec-alist} is
+used as a basis for all of that character's substitutions.
+@item
+The order of specifications in @var{template} need not correspond to
+the order of associations in @var{spec-alist}.
+@end itemize
+
+The optional argument @var{only-present} indicates how to handle
+specification characters in @var{template} that are not found in
+@var{spec-alist}. If it is @code{nil} or omitted, the function
+signals an error. Otherwise, those format specifications and any
+occurrences of @samp{%%} in @var{template} are left verbatim in the
+output, including their text properties, if any.
+@end defun
+
+The syntax of format specifications accepted by @code{format-spec} is
+similar, but not identical, to that accepted by @code{format}. In
+both cases, a format specification is a sequence of characters
+beginning with @samp{%} and ending with an alphabetic letter such as
+@samp{s}.
+
+Unlike @code{format}, which assigns specific meanings to a fixed set
+of specification characters, @code{format-spec} accepts arbitrary
+specification characters and treats them all equally. For example:
+
+@example
+@group
+(setq my-site-info
+ (list (cons ?s system-name)
+ (cons ?t (symbol-name system-type))
+ (cons ?c system-configuration)
+ (cons ?v emacs-version)
+ (cons ?e invocation-name)
+ (cons ?p (number-to-string (emacs-pid)))
+ (cons ?a user-mail-address)
+ (cons ?n user-full-name)))
+
+(format-spec "%e %v (%c)" my-site-info)
+ @result{} "emacs 27.1 (x86_64-pc-linux-gnu)"
+
+(format-spec "%n <%a>" my-site-info)
+ @result{} "Emacs Developers <emacs-devel@@gnu.org>"
+@end group
+@end example
+
+A format specification can include any number of the following flag
+characters immediately after the @samp{%} to modify aspects of the
+substitution.
+
+@table @samp
+@item 0
+This flag causes any padding specified by the width to consist of
+@samp{0} characters instead of spaces.
+
+@item -
+This flag causes any padding specified by the width to be inserted on
+the right rather than the left.
+
+@item <
+This flag causes the substitution to be truncated on the left to the
+given width, if specified.
+
+@item >
+This flag causes the substitution to be truncated on the right to the
+given width, if specified.
+
+@item ^
+This flag converts the substituted text to upper case (@pxref{Case
+Conversion}).
+
+@item _
+This flag converts the substituted text to lower case (@pxref{Case
+Conversion}).
+@end table
+
+The result of using contradictory flags (for instance, both upper and
+lower case) is undefined.
+
+As is the case with @code{format}, a format specification can include
+a width, which is a decimal number that appears after any flags. If a
+substitution contains fewer characters than its specified width, it is
+padded on the left:
+
+@example
+@group
+(format-spec "%8a is padded on the left with spaces"
+ '((?a . "alpha")))
+ @result{} " alpha is padded on the left with spaces"
+@end group
+@end example
+
+Here is a more complicated example that combines several
+aforementioned features:
+
+@example
+@group
+(setq my-battery-info
+ (list (cons ?p "73") ; Percentage
+ (cons ?L "Battery") ; Status
+ (cons ?t "2:23") ; Remaining time
+ (cons ?c "24330") ; Capacity
+ (cons ?r "10.6"))) ; Rate of discharge
+
+(format-spec "%>^-3L : %3p%% (%05t left)" my-battery-info)
+ @result{} "BAT : 73% (02:23 left)"
+
+(format-spec "%>^-3L : %3p%% (%05t left)"
+ (cons (cons ?L "AC")
+ my-battery-info))
+ @result{} "AC : 73% (02:23 left)"
+@end group
+@end example
+
+As the examples in this section illustrate, @code{format-spec} is
+often used for selectively formatting an assortment of different
+pieces of information. This is useful in programs that provide
+user-customizable format strings, as the user can choose to format
+with a regular syntax and in any desired order only a subset of the
+information that the program makes available.
+
@node Case Conversion
@section Case Conversion in Lisp
@cindex upper case
of another buffer.
* Decompression:: Dealing with compressed data.
* Base 64:: Conversion to or from base 64 encoding.
-* Interpolated Strings:: Formatting Customizable Strings.
* Checksum/Hash:: Computing cryptographic hashes.
* GnuTLS Cryptography:: Cryptographic algorithms imported from GnuTLS.
* Parsing HTML/XML:: Parsing HTML and XML.
is optional, and the URL variant of base 64 encoding is used.
@end defun
-
-@node Interpolated Strings
-@section Formatting Customizable Strings
-
-It is, in some circumstances, useful to present users with a string to
-be customized that can then be expanded programmatically. For
-instance, @code{erc-header-line-format} is @code{"%n on %t (%m,%l)
-%o"}, and each of those characters after the percent signs are
-expanded when the header line is computed. To do this, the
-@code{format-spec} function is used:
-
-@defun format-spec format specification &optional only-present
-@var{format} is the format specification string as in the example
-above. @var{specification} is an alist that has elements where the
-@code{car} is a character and the @code{cdr} is the substitution.
-
-If @var{only-present} is @code{nil}, errors will be signaled if a
-format character has been used that's not present in
-@var{specification}. If it's non-@code{nil}, that format
-specification is left verbatim in the result.
-@end defun
-
-Here's a trivial example:
-
-@example
-(format-spec "su - %u %l"
- `((?u . ,(user-login-name))
- (?l . "ls")))
- @result{} "su - foo ls"
-@end example
-
-In addition to allowing padding/limiting to a certain length, the
-following modifiers can be used:
-
-@table @asis
-@item @samp{0}
-Pad with zeros instead of the default spaces.
-
-@item @samp{-}
-Pad to the right.
-
-@item @samp{^}
-Use upper case.
-
-@item @samp{_}
-Use lower case.
-
-@item @samp{<}
-If the length needs to be limited, remove characters from the left.
-
-@item @samp{>}
-Same as previous, but remove characters from the right.
-@end table
-
-If contradictory modifiers are used (for instance, both upper and
-lower case), then what happens is undefined.
-
-As an example, @samp{"%<010b"} means ``insert the @samp{b} expansion,
-but pad with leading zeros if it's less than ten characters, and if
-it's more than ten characters, shorten by removing characters from the
-left.''
-
-
@node Checksum/Hash
@section Checksum/Hash
@cindex MD5 checksum
(defun format-spec (format specification &optional only-present)
"Return a string based on FORMAT and SPECIFICATION.
-FORMAT is a string containing `format'-like specs like \"su - %u %k\",
-while SPECIFICATION is an alist mapping from format spec characters
-to values.
+FORMAT is a string containing `format'-like specs like \"su - %u %k\".
+SPECIFICATION is an alist mapping format specification characters
+to their substitutions.
For instance:
(format-spec \"su - %u %l\"
- `((?u . ,(user-login-name))
+ \\=`((?u . ,(user-login-name))
(?l . \"ls\")))
-Each format spec can have modifiers, where \"%<010b\" means \"if
-the expansion is shorter than ten characters, zero-pad it, and if
-it's longer, chop off characters from the left side\".
+Each %-spec may contain optional flag and width modifiers, as
+follows:
-The following modifiers are allowed:
+ %<flags><width>character
-* 0: Use zero-padding.
-* -: Pad to the right.
-* ^: Upper-case the expansion.
-* _: Lower-case the expansion.
-* <: Limit the length by removing chars from the left.
-* >: Limit the length by removing chars from the right.
+The following flags are allowed:
-Any text properties on a %-spec itself are propagated to the text
-that it generates.
+* 0: Pad to the width, if given, with zeros instead of spaces.
+* -: Pad to the width, if given, on the right instead of the left.
+* <: Truncate to the width, if given, on the left.
+* >: Truncate to the width, if given, on the right.
+* ^: Convert to upper case.
+* _: Convert to lower case.
-If ONLY-PRESENT, format spec characters not present in
-SPECIFICATION are ignored, and the \"%\" characters are left
-where they are, including \"%%\" strings."
+The width modifier behaves like the corresponding one in `format'
+when applied to %s.
+
+For example, \"%<010b\" means \"substitute into the output the
+value associated with ?b in SPECIFICATION, either padding it with
+leading zeros or truncating leading characters until it's ten
+characters wide\".
+
+Any text properties of FORMAT are copied to the result, with any
+text properties of a %-spec itself copied to its substitution.
+
+ONLY-PRESENT indicates how to handle %-spec characters not
+present in SPECIFICATION. If it is nil or omitted, emit an
+error; otherwise leave those %-specs and any occurrences of
+\"%%\" in FORMAT verbatim in the result, including their text
+properties, if any."
(with-temp-buffer
(insert format)
(goto-char (point-min))
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