incdec.pm: Fix an issue where first number wasn't incremented backwards

When `$start_position` was 0 going backwards, the `sed` test command
line failed to increment the first number in the line.

Can't figure out exactly what was wrong, but there seemed to be a
problem with using the `\G` assertion for that test case. Decided to
remove `\G` when `$start_position` position is 0 to work around the
problem. Not sure if there's a more concise solution to this that
wouldn't require me to have two separate subtitution lines.

Also simplified the substitution regular expression. It turns out I
didn't need the first capture group, and it was incorrect in matching
/[^-\d]*/ because we really wanted /(?!-?\d+)/, not either or of the
characters in the group. Completely removing it still allows everything
to work. Don't remember if I added that when I was still using the
substringing algorithm, but whatever happens, it's not necessary now.

3 files changed, 40 insertions, 10 deletions

diff --git a/incdec.bash b/incdec.bash
index 9950f33..d00c140 100644
--- a/incdec.bash
+++ b/incdec.bash
@@ -58,8 +58,14 @@ sub incdec {
 		}
 	}
 
-	pos($line) = $start_position;
-	$line =~ s/\G([^-\d]*)(-?\d+)/$1 . ($2 + $increment_by)/e;
+	# Using `\G` when `pos` is 0 seems to cause occasional missed substitutions.
+	if ($start_position > 0) {
+		pos($line) = $start_position;
+		$line =~ s/\G(-?\d+)/$1 + $increment_by/e;
+	}
+	else {
+		$line =~ s/(-?\d+)/$1 + $increment_by/e;
+	}
 
 	return $line;
 }
diff --git a/incdec.pm b/incdec.pm
index b53d27c..5d0783e 100644
--- a/incdec.pm
+++ b/incdec.pm
@@ -55,8 +55,14 @@ sub incdec {
 		}
 	}
 
-	pos($line) = $start_position;
-	$line =~ s/\G([^-\d]*)(-?\d+)/$1 . ($2 + $increment_by)/e;
+	# Using `\G` when `pos` is 0 seems to cause occasional missed substitutions.
+	if ($start_position > 0) {
+		pos($line) = $start_position;
+		$line =~ s/\G(-?\d+)/$1 + $increment_by/e;
+	}
+	else {
+		$line =~ s/(-?\d+)/$1 + $increment_by/e;
+	}
 
 	return $line;
 }
diff --git a/t/100-increment-decrement.t b/t/100-increment-decrement.t
index 883a9c2..6fada9b 100644
--- a/t/100-increment-decrement.t
+++ b/t/100-increment-decrement.t
@@ -104,12 +104,6 @@ is(
 );
 
 is(
-	incdec::incdec("sed -n '39,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ", 1, 3, 1),
-	"sed -n '40,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ",
-	'increments the first integer with point at position 3 backward'
-);
-
-is(
 	incdec::incdec('test -1 ', 1, 7, 1),
 	'test 0 ',
 	'increments the negative integer with point at position 7 backward'
@@ -169,4 +163,28 @@ is(
 	'decrements the second integer by 2 with point at position 9 backward'
 );
 
+is(
+	incdec::incdec("sed -n '39,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ", 1, 3, 1),
+	"sed -n '40,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ",
+	'increments the first integer with point at position 3 backward'
+);
+
+is(
+	incdec::incdec("sed -n '39,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ", 1, 8, 1),
+	"sed -n '40,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ",
+	'increments the first integer with point at position 8 backward'
+);
+
+is(
+	incdec::incdec("sed -n '39,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ", -1, 10, 1),
+	"sed -n '38,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ",
+	'decrements the first integer with point at position 10 backward'
+);
+
+is(
+	incdec::incdec("sed -n '39,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ", 1, 3, 0),
+	"sed -n '40,54p' Alice\'s\ Adventures\ in\ Wonderland.txt ",
+	'increments the first integer with point at position 3'
+);
+
 done_testing;