Add an example of trap command (#1826)

* Begin writing document for PCRE

Started writing learnxinyminutes document for PCRE to cover general purpose regular expressions. 
Added introduction and a couple of details.

* Change introductory example for regex

The old example was incorrect. It's replaced with a simple one.

* Add some more introductory text

* Add first example

* Added more example and a table for proper formatting

* Add few more examples

* Formatting

* Improve example

* Edit description of character classes

* Add a way to test regex

Add https://regex101.com/ web application to test the regex provided in example.

* Add example of trap command 

trap is a very important command to intercept a fatal signal, perform cleanup, and then exit gracefully. It needs an entry in this document.

Here a simple and most common example of using trap command i.e. cleanup upon receiving signal is added.

* Revert "Add example of trap command"

* Add an example of trap command

`trap` is a very important command to intercept a fatal signal, perform cleanup, and then exit gracefully. It needs an entry in this document.

Here a simple and most common example of using `trap` command i.e. cleanup upon receiving signal is added.

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+---
+language: PCRE
+filename: pcre.txt
+contributors:
+    - ["Sachin Divekar", "http://github.com/ssd532"]
+    
+---
+
+A regular expression (regex or regexp for short) is a special text string for describing a search pattern. e.g. to extract domain name from a string we can say `/^[a-z]+:/` and it will match `http:` from `http://github.com/`.  
+
+PCRE (Perl Compatible Regular Expressions) is a C library implementing regex. It was written in 1997 when Perl was the de-facto choice for complex text processing tasks. The syntax for patterns used in PCRE closely resembles Perl. PCRE syntax is being used in many big projects including PHP, Apache, R to name a few.
+
+
+There are two different sets of metacharacters:
+* Those that are recognized anywhere in the pattern except within square brackets
+```
+  \      general escape character with several uses
+  ^      assert start of string (or line, in multiline mode)
+  $      assert end of string (or line, in multiline mode)
+  .      match any character except newline (by default)
+  [      start character class definition
+  |      start of alternative branch
+  (      start subpattern
+  )      end subpattern
+  ?      extends the meaning of (
+         also 0 or 1 quantifier
+         also quantifier minimizer
+  *      0 or more quantifier
+  +      1 or more quantifier
+         also "possessive quantifier"
+  {      start min/max quantifier
+```
+
+* Those that are recognized within square brackets. Outside square brackets. They are also called as character classes.
+ 
+```
+ 
+  \      general escape character
+  ^      negate the class, but only if the first character
+  -      indicates character range
+  [      POSIX character class (only if followed by POSIX syntax)
+  ]      terminates the character class
+  
+```  
+
+PCRE provides some generic character types, also called as character classes. 
+```
+  \d     any decimal digit
+  \D     any character that is not a decimal digit
+  \h     any horizontal white space character
+  \H     any character that is not a horizontal white space character
+  \s     any white space character
+  \S     any character that is not a white space character
+  \v     any vertical white space character
+  \V     any character that is not a vertical white space character
+  \w     any "word" character
+  \W     any "non-word" character
+```
+
+## Examples
+
+We will test our examples on following string `66.249.64.13 - - [18/Sep/2004:11:07:48 +1000] "GET /robots.txt HTTP/1.0" 200 468 "-" "Googlebot/2.1"`. It is a standard Apache access log.
+
+| Regex | Result          | Comment |
+| :---- | :-------------- | :------ |
+| GET   | GET | GET matches the characters GET literally (case sensitive) |
+| \d+.\d+.\d+.\d+ | 66.249.64.13 | `\d+` match a digit [0-9] one or more times defined by `+` quantifier, `\.` matches `.` literally |
+| (\d+\.){3}\d+ | 66.249.64.13 | `(\d+\.){3}` is trying to match group (`\d+\.`) exactly three times. |
+| \[.+\] | [18/Sep/2004:11:07:48 +1000] | `.+` matches any character (except newline), `.` is any character |
+| ^\S+ | 66.249.64.13 | `^` means start of the line, `\S+` matches any number of non-space characters |
+| \+[0-9]+ | +1000 | `\+` matches the character `+` literally. `[0-9]` character class means single number. Same can be achieved using `\+\d+` |
+
+All these examples can be tried at https://regex101.com/ 
+
+1. Copy the example string in `TEST STRING` section
+2. Copy regex code in `Regular Expression` section 
+3. The web application will show the matching result
+
+
+## Further Reading
+
+