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.
This commit is contained in:
Sachin Divekar 2016-06-26 18:08:05 +05:30 committed by ven
parent d4aa031d55
commit d1216a4253
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@ -272,6 +272,9 @@ grep -c "^foo.*bar$" file.txt
# and not the regex, use fgrep (or grep -F) # and not the regex, use fgrep (or grep -F)
fgrep "foobar" file.txt fgrep "foobar" file.txt
# trap command allows you to execute a command when a signal is received by your script.
# Here trap command will execute rm if any one of the three listed signals is received.
trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM
# Read Bash shell builtins documentation with the bash 'help' builtin: # Read Bash shell builtins documentation with the bash 'help' builtin:
help help

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pcre.html.markdown Normal file
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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