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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘RegEx’ Category

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Delphi TRegExOption: Where is description of roNotEmpty option? What does this option do? – Jacek Laskowski – Google+

Posted by jpluimers on 2020/12/10

I really dislike using regular expressions, mainly because every time I bump into code using them either:

  • I cannot decipher them any more
  • It is used for things not suites for (like parsing JSON or XML: please don’t!)

For more background on when NOT to use regular expressions, remember they describe a regular grammar, and can only me implemented by a finite state machine (a state machine that can be exactly one state out of a set of finite states).

As soon as you need to parse something that needs multiple states at once, or the number of states becomes infinite,

Some background reading:

Read the rest of this entry »

Posted in Delphi, Development, RegEx, Software Development | Leave a Comment »

shell – How do I grep for multiple patterns with pattern having a pipe character? – Unix & Linux Stack Exchange

Posted by jpluimers on 2020/10/27

Since I keep forgetting this – especially because I cannot remember the “why”: [WayBack] shell – How do I grep for multiple patterns with pattern having a pipe character? – Unix & Linux Stack Exchange by “user unknown“.

The -E means using Regular expression: POSIX extended – Wikipedia.

egrep "foo|bar" *.txt

or

grep "foo\|bar" *.txt
grep -E "foo|bar" *.txt

selectively citing the man page of gnu-grep:

   -E, --extended-regexp
          Interpret PATTERN as an extended regular expression (ERE, see below).  (-E is specified by POSIX.)

Matching Control
   -e PATTERN, --regexp=PATTERN
          Use PATTERN as the pattern.  This can be used to specify multiple search patterns, or to protect  a  pattern
          beginning with a hyphen (-).  (-e is specified by POSIX.)

(…)

   grep understands two different versions of regular expression syntax: “basic” and “extended.”  In  GNU grep,  there
   is  no  difference  in  available  functionality  using  either  syntax.   In  other implementations, basic regular
   expressions are less powerful.  The following description applies to extended regular expressions; differences  for
   basic regular expressions are summarized afterwards.

In the beginning I didn’t read further, so I didn’t recognize the subtle differences:

Basic vs Extended Regular Expressions
   In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead  use  the
   backslashed versions \?, \+, \{, \|, \(, and \).

I always used egrep and needlessly parens, because I learned from examples. Now I learned something new. :)

–jeroen

Posted in Development, RegEx, Software Development | Leave a Comment »

sed double expression: match, replace in one line, overwrite file

Posted by jpluimers on 2020/04/15

A while ago, I needed to conditionally replace in files, so I used sed and a regular expression, though usually I dislike those.

However, since the system had a very basic install, there was not much choice.

Luckily back then, my Google foo returned these:

This allowed me to do a double expression (the first matches a pattern, the second performs the actual replacement within the matching lines).

In case my Google foo in the future fails:

## https://robots.thoughtbot.com/sed-102-replace-in-place
## -i causes no backup to be saved, but does in-place replacement
## since we run under git, we can always restore
## combined with a double expression (the first matches, the second executes) this is very powerful
sed -i -e '/#.*AVOID_DAILY_AUTOCOMMITS=.*$/s/^.//' /etc/etckeeper/etckeeper.conf && git diff | more

–jeroen

Posted in Development, RegEx, Software Development | Leave a Comment »

GExperts: searching for case-insensitive “T*List.Create” but not “TStringList.Create”

Posted by jpluimers on 2019/05/02

Just learned that partial exclusion can be done with the case-insensitive GExperts Grep Search like this:

T[^s][^t][^r][^i][^n][^g].*List.*\.Create

This will skip TStringList.Create, but matches TMyList.Create.

I’d rather have done something like this, but the Delphi RegEx does not support negative lookbehind:

^ *[a-zA-Z0-9_]* *: *T(<!string)[a-zA-Z0-9_]*ListO? *;$

So the alternative is to search for this:

^ *[a-zA-Z0-9_]* *: *T[a-zA-Z0-9_]*ListO? *;$

then exclude all the case insensitive TStringList entries from it, however GExperts did not support that at the time of writing.

This is an intermediate that works for some of the times:

^ *[a-zA-Z0-9_]* *: *T[^s][^t][^r][^i][^n][^g][a-zA-Z0-9_]*ListO? *;$

–jeroen

^ *[a-zA-Z0-9_]* *: *T[^s][^t][^r][^i][^n][^g][a-zA-Z0-9_]*ListO? *$;
^ *[a-zA-Z0-9_]*: .T[a-zA-Z0-9_]*ListO? *;$;
^ *[a-zA-Z0-9_]* *: *T(?!string)[a-zA-Z0-9_]*ListO? *;$

Posted in Delphi, Development, GExperts, RegEx, Software Development | Leave a Comment »

RegExr: Online Regular Expression Testing Tool

Posted by jpluimers on 2018/11/19

[WayBack] Kevlin Henney – Google+ reminded me of RegExr. Then I found out I collected the below links in 2010, but never published them. Hopefully a few still exist. Let’s see… yup: all do, but archived just in case.

Some other links (that show how much the landscape changed in 8.5 years time):

–jeroen

Posted in Development, RegEx, Software Development | Leave a Comment »

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