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Add repetitions meta character

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Zeeshan Ahmed 2017-07-23 12:06:54 +01:00
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@ -54,18 +54,18 @@ The meta character are as follows:
|Meta character|Description|
|:----:|----|
|<b>.</b>|Period matches any single character except a line break.|
|<b>[ ]</b>|Character class. Matches any character contained between the square brackets.|
|<b>[^ ]</b>|Negated character class. Matches any character that is not contained between the square brackets|
|<b>*</b>|Matches 0 or more repetitions of the preceding symbol.|
|<b>+</b>|Matches 1 or more repetitions of the preceding symbol.
|<b>?</b>|Makes the preceding symbol optional.|
|<b>{n}</b>|Braces. Matches “n” repetitions of the preceding symbol.|
|<b>(xyz)</b>|Character group. Matches the characters xyz in that exact order.|
|<b>&#124;</b>|Alternation. Matches either the characters before or the characters after the symbol.|
|<b>&#92;</b>|Escapes the next character. This allows you to match reserved characters <code>[ ] ( ) { } . * + ? ^ $ \ &#124;</code>|
|<b>^</b>|Matches the beginning of the input.|
|<b>$</b>|Matches the end of the input.|
|.|Period matches any single character except a line break.|
|[ ]|Character class. Matches any character contained between the square brackets.|
|[^ ]|Negated character class. Matches any character that is not contained between the square brackets|
|*|Matches 0 or more repetitions of the preceding symbol.|
|+|Matches 1 or more repetitions of the preceding symbol.
|?|Makes the preceding symbol optional.|
|{n}|Braces. Matches “n” repetitions of the preceding symbol.|
|(xyz)|Character group. Matches the characters xyz in that exact order.|
|&#124;|Alternation. Matches either the characters before or the characters after the symbol.|
|&#92;|Escapes the next character. This allows you to match reserved characters <code>[ ] ( ) { } . * + ? ^ $ \ &#124;</code>|
|^|Matches the beginning of the input.|
|$|Matches the end of the input.|
## 2.1 Full stop
@ -87,6 +87,12 @@ expression `[Tt]he` means: an uppercase `T` or lowercase `t`, followed by the le
"[Tt]he" => <strong><u>The</u></strong> car parked in <strong><u>the</u></strong> garage.
</pre>
Just like above example the regular expression `ar[.]` means: an lowercase character `a`, followed by letter `r`, followed by any character.
<pre>
"ar[.]" => The car p<strong><u>ark</u></strong>ed in the g<strong><u>ara</u></strong>ge.
</pre>
### 2.2.1 Negated character set
In general the caret symbol represents the start of the string, but when it is typed after the opening square bracket it negates the
@ -98,11 +104,49 @@ the letter `r`.
</pre>
### 2.2.2 Repeating character set
## 2.3 Repetitions
We can repeat a character class by using `+`, `*` or `?` operators. For example the regular expression `[a-z]+` means: any number of
lowercase letters in a row.
Following meta characters `+`, `*` or `?` are used to specify how many times a subpattern can occurs. These meta characters act
differently in different situations.
### 2.3.1 The Star
The symbol `*` matches zero or more repetitions of the preceding matcher. The regular expression `a*` means: zero or more repetitions
of preceding lowercase character `a`. But if it apperas after a character set or class that it finds the repetitions of the whole
character set. For example the regular expression `[a-z]*` means: any number of lowercase letters in a row.
<pre>
"[a-z]+" => <strong><u>The</u></strong> <strong><u>car</u></strong> <strong><u>parked</u></strong> <strong><u>in</u></strong> <strong><u>the</u></strong> <strong><u>garage</u></strong>.
"[a-z]*" => <strong><u>The</u></strong> <strong><u>car</u></strong> <strong><u>parked</u></strong> <strong><u>in</u></strong> <strong><u>the</u></strong> <strong><u>garage</u></strong> #21.
</pre>
The `*` symbol can be used with the meta character `.` to match any string of characters `.*`. The `*` symbol can be used with the
whitespace character `\s` to match a string of whitespace characters. For example the expression `\s*cat\s*` means: zero or more
spaces, followed by lowercase character `c`, followed by lowercase character `a`, followed by lowercase character `t`, followed by
zero or more spaces.
<pre>
"\s*cat\s*" => The fat<strong><u> cat </u></strong>sat on the <strong><u>cat</u></strong>.
</pre>
### 2.3.2 The Plus
The symbol `+` matches one or more repetitions of the preceding character. For example the regular expression `c.+t` means: lowercase
letter `c`, followed by any number of character, followed by the lowercase character `t`.
<pre>
"c.+t" => The fat <strong><u>cat sat on the mat</u></strong>.
</pre>
### 2.3.3 The Question Mark
In regular expression the meta character `?` makes the preceding character optional. This symbol matches zero or more repetitions of
the preceding character. For example the regular expression `[T]?he` means: Optional the uppercase letter `T`, followed by the lowercase
character `h`, followed by the lowercase character `e`.
<pre>
"[T]he" => <strong><u>The</u></strong> car is parked in the garage.
</pre>
<pre>
"[T]?he" => <strong><u>The</u></strong> car is parked in t<strong><u>he</u></strong> garage.
</pre>