Per #4: Add implicit parenthesized expression

README.org

@@ -130,6 +130,13 @@
 ** ~ialign-initial-repeat~
  Default state of ~repeat~ argument passed to ~align-regexp~.
 
+ The default value is ~nil~.
+** ~ialign-implicit-regexp~
+ String to prepend to the regexp, if the regex doesn't have a subgroup.
+
+ Set this to ~\\(\\s-*\\)~ to implicitly prepend that string to
+ the regexp, if the regexp doesn't contain a subgroup expression.
+
  The default value is ~nil~.
 * License
  This program is free software: you can redistribute it and/or modify

ialign.el

@@ -101,6 +101,20 @@ or equal to this, otherwise do not update."
  :group 'ialign
  :type 'boolean)
 
+(defcustom ialign-implicit-regexp nil
+ "String to prepend to the regexp, if the regexp doesn't have a subgroup.
+
+`align-regexp' expects the align regexp to contain a
+parenthesized subexpression whose characters are replaced.
+However, it adds such an expression automatically to the regexp
+if necessary. This option allows you to specify the string to
+implicitly prepend to the string in case there's no group
+subexpression."
+ :group 'ialign
+ :type '(choice (const :tag "Don't add implicit group" nil)
+ (const :tag "Whitespace" "\\(\\s-*\\)")
+ string))
+
 (defvaralias 'ialign-initial-spacing 'ialign-default-spacing)
 
 (defvar ialign--buffer nil)
@@ -193,11 +207,19 @@ help"))))
 (defun ialign--align ()
  "Revert the current region, then align it."
  (ialign--revert)
- (ialign--with-region-narrowed
- (let ((case-fold-search ialign--case-fold-search)
- (indent-tabs-mode (ialign--enable-tabs-p)))
- (align-regexp (point-min) (point-max) ialign--regexp
- ialign--group ialign--spacing ialign--repeat))))
+ (let ((reg ialign--regexp))
+ (when (and (null (string-match-p (regexp-quote "\\(") reg))
+ (stringp ialign-implicit-regexp)
+ (= 1 ialign--group))
+ (setq reg (concat ialign-implicit-regexp reg))
+ (setq ialign--error "Using implicit regexp")
+ (when (minibufferp)
+ (ialign--update-minibuffer-prompt)))
+ (ialign--with-region-narrowed
+ (let ((case-fold-search ialign--case-fold-search)
+ (indent-tabs-mode (ialign--enable-tabs-p)))
+ (align-regexp (point-min) (point-max) reg
+ ialign--group ialign--spacing ialign--repeat)))))
 
 (defun ialign--undo (beg end orig)
  "Delete region BEG END and insert ORIG at BEG.