Powershell regex replace only first hit

Question

I'm trying to use a regular expression to replace the first character after a single hit, while using PowerShell.

No matter how I try, I can't seem to make it work. Here's what I'm talking about:

Code:

$info = 'AB/F/*ZXCVBN/MTF/ ---'

$regex = [REGEX]'/*'
$regex.Replace($info,"/C",1)

$regex

Output:

/CAB/F/*ZXCVBN/MTF/ ---

I'm simply trying to replace the /F in the expression with /C, but it fails every time.

I'm using /* since I don't really know what character will I find after the first / but that's what I want to replace in the end of the day.

I pretty sure this will be pretty simple but, as you can see, I'm, just not familiar enough with regular expressions.

have you tried escaping * and use /\* instead?

Federico Piazza
– Federico Piazza

2014-09-10 16:25:51 +00:00
Commented Sep 10, 2014 at 16:25 — Federico Piazza
– Federico Piazza, Commented Sep 10, 2014 at 16:25

TheMadTechnician · Accepted Answer · 2014-09-10 17:07:22Z

1

Ok, rather than just a comment I guess I'll add an answer. You can use a negative lookbehind to make sure that there are no /'s before what you are matching, so it will only match the first one. Also, as Noah stated the * is not a wildcard, . is. This will match any / plus 1 character that does not have another / anywhere before it in the string:

"(?<!/.*)/."

So in context to your code, it would look like this:

$info = 'AB/F/*ZXCVBN/MTF/ ---'
$regex = [REGEX]"(?<!/)/."
$regex.Replace($info,"/C",1)

Those lines will output:

AB/C/*ZXCVBN/MTF/ ---

Edit: RegEx broken down at RegEx101: http://regex101.com/r/tI7oN1/1

edited Sep 10, 2014 at 17:07

answered Sep 10, 2014 at 16:54

TheMadTechnician

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1 Comment

NianRen Over a year ago

Brilliant! Thank you so much! Pretty obvious I have a lot of reading/experimenting to do with regular expressions... Thank you to everyone that pitched in, it is truly appreciated!

Brian Stephens · Accepted Answer · 2014-09-10 16:54:17Z

0

$info = 'AB/F/*ZXCVBN/MTF/ ---'
$regex = [REGEX]'^([^/]*)/[a-zA-Z]'
$regex.Replace($info,"$1/C",1)
$regex

^([^/]*) - this looks for anything but slashes at the beginning, captured in a group

/[a-zA-Z] - then a slash followed by a letter

The replacement puts back whatever was matched by the first group, and adds /C

answered Sep 10, 2014 at 16:54

Brian Stephens

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Comments

walid toumi · Accepted Answer · 2014-09-10 16:58:22Z

0

$info = 'AB/F/*ZXCVBN/MTF/ ---' 
$regex = [REGEX]'/.'
 $regex.Replace($info,"/C",1)

Or simply:

$info -replace '^(.*?)/.','$1/C'

edited Sep 10, 2014 at 16:58

answered Sep 10, 2014 at 16:45

walid toumi

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mjolinor · Accepted Answer · 2014-09-10 18:02:05Z

0

If you don't want to use the regex, you can accomplish the same thing with -split and -join:

$info = 'AB/F/*ZXCVBN/MTF/ ---'
$info -split '/.',2 -join '/C'

AB/C/*ZXCVBN/MTF/ ---

The ,2 will stop the split after the first match (2 elements). Then re-join the elements with /C.

answered Sep 10, 2014 at 18:02

mjolinor

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Comments

Noah Sparks · Accepted Answer · 2014-09-10 19:30:09Z

0

You are misunderstanding * in regex. It is not a wildcard character.

The star in regex means capture 0 or more of the preceding items in the expression. Wildcard in regex is actually a period.

edited Sep 10, 2014 at 19:30

answered Sep 10, 2014 at 16:27

Noah Sparks

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2 Comments

TheMadTechnician Over a year ago

Nah Noah, don't do that, just use a negative lookbehind: $string -replace "(?<!/.*)/.", "/C"

Noah Sparks Over a year ago

I tried to think of some ways for a minute to do it in one expression and was drawing blanks. Good idea.

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