Unexpected result from PowerShell regex

Question

I am trying to identify errors in a log file. The application uses five uppercase letters followed by three digits followed by 'E' as an error code. The error code is followed by a non-word character. I was identifying cases with:

$errors=Select-string -Path "logfile.txt" -Pattern "[A-Z]{5}[0-9]{3}E\W"

However the remainder of the content now includes

ab1bea8a-a00e-4211-b1db-2facecfd725e.

Which is being matched by the regex and flagged as an error. I changed the regex to

\p{Lu}{5}[0-9]{3}E\W

(which I expected to match five upper case characters), but why does it still match the non-error lower case pattern?

You may make \p{Lu} insensitive to the outer regex settings by placing it into a modifier group like (?-i:\p{Lu}{5}[0-9]{3}E\W) — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 9, 2018 at 9:37

Tomalak · Accepted Answer · 2018-07-09 09:38:51Z

4

The "case-insensitive" regex flag is set by Select-String, which makes \p{Lu} case-insensitive, just as it does with [A-Z].

Try adding the -CaseSensitive parameter to the command.

You can confirm this by running some .NET code, for example in LINQPad:

(new Regex(@"\p{Lu}", RegexOptions.IgnoreCase)).IsMatch("a")

edited Jul 9, 2018 at 9:38

answered Jul 9, 2018 at 9:28

Tomalak

340k68 gold badges547 silver badges635 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Wiktor Stribiżew Over a year ago

You may also include this link to the answer. The detail about Select-String case-insensitive matching behavior is not easy find in the docs, although the -CaseSensitive option should hint at that by all means. Also, if one wants to mix case sensitivity options, it would be a good idea to add an example with modifier groups (not the case here, but would be good for generic cases).

Tomalak Over a year ago

Agreed, I thought about including that, but it wasn't a direct requirement of the question. And then I saw your comment, so that was taken care of anyway. :)

Ansgar Wiechers · Accepted Answer · 2018-07-14 13:08:13Z

PowerShell regular expression matching is case-insensitive by default. There are several ways for making matches case-sensitive, though.

Add the -CaseSensitive switch when using the Select-String cmdlet:

-CaseSensitive

Makes matches case-sensitive. By default, matches are not case-sensitive.

C:\> 'abc' | Select-String -Pattern 'A'

abc

C:\> 'ABC' | Select-String -Pattern 'A'

ABC

C:\> 'abc' | Select-String -Pattern 'A' -CaseSensitive    # ← no match here
C:\> 'ABC' | Select-String -Pattern 'A' -CaseSensitive

ABC

Use the case-sensitive version of the regular expression matching operators:

By default, all comparison operators are case-insensitive. To make a comparison operator case-sensitive, precede the operator name with a c. For example, the case-sensitive version of -eq is -ceq. To make the case-insensitivity explicit, precede the operator with an i. For example, the explicitly case-insensitive version of -eq is -ieq.
```
C:\> 'abc' -match 'A'
True
C:\> 'ABC' -match 'A'
True
C:\> 'abc' -cmatch 'A'    # ← no match here
False
C:\> 'ABC' -cmatch 'A'
True
```
Force a case-sensitive match by adding a miscellaneous construct ((?...), not to be confused with non-capturing groups (?:...)) with the inverted "case-insensitive" regex option to the regular expression (this works with both Select-String cmdlet and -match operator):
```
C:\> 'abc' | Select-String -Pattern '(?-i)A'    # ← no match here
C:\> 'ABC' | Select-String -Pattern '(?-i)A'

ABC

C:\> 'abc' -match '(?-i)A'    # ← no match here
False
C:\> 'ABC' -match '(?-i)A'
True
```

Collectives™ on Stack Overflow

Unexpected result from PowerShell regex

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related