Regex
m/(b?)-/ [here no g flag]
Sample input
a-b-
Current matching
-
Desired matching
b-
My doubt is about why without a global flag b- is not matched
(b?) is optionally matching literal b
"-" is matched literal -
finally, I get only - this one?
$ echo "a-b-" | sed 's/\(b\?\)-/x\u\1/g'
axxB
$
$ echo "a-b-" | sed 's/\(b\?\)-/x\u\1/'
axb-
In the last sed replacement, the first occurrence of - is replaced by x what about the b-
Without the global flag regex will return first match only. Since your regex is b?- with optional b therefore it returns first - from input a-b-.
If you still want to match b- even though - appears first then you can use this regex with a negative lookahead:
b?-(?!.*b-)
This will match optional b followed by a - only if there is no b- on right hand side since we are using a negative lookahead.
If there is no b- then it will match first -.