Regular Expression
Regular expression is a method of pattern matching. It allows you to define patterns of texts and search for it in other texts, no matter the complexity or the amount of text there is to process.
Using regular expression is considered a more efficient solution, when there’s complexity in searching for a text that you want to find. It is also called as regex.
Keep in mind that regular expression is supported in almost every programming language. If you were to face a situation when there’s complexity in searching a text, take a look at how the systax looks like other programming languages and utilize them.
Getting Started with Regular Expressions
Learning the fundamentals before diving into the advanced areas will help you grasp the concept.
1. Literal Strings
This is a simple and a straightforward matching, that it simply matches “hello” with “hello”. This is yet another way to using regex, but a simple search as well.
2. Metacharacters
It uses special characters(symbols other than alpha-numeric) and each gives a special meaning in regular expression.
.= Dot- Match any single character; It must include any single character.
h-y=heyorhoyorhuy…
^= Caret- Aka Anchor. It matches any string that starts with
^. - Eg:
^abc- It looks for string that starts withabc.
- Aka Anchor. It matches any string that starts with
$= Dollar Sign- It matches any string that ends with
$. - Eg:
abc$- It looks for string that ends withabc.
- It matches any string that ends with
*= Asterisk- It matches zero or more string; It can skip matching characters
- Eg:
he*=hey,hello,heat… - It can match any number of characters that starts after
he.... - In case, it can’t match any, it will look for
heonly, since*it’s not necessary to add a character anyway.
+= Plus Sign- It matches one or more characters; Similar to
.(dot), but it must match atleast one character. - Eg:
+uck=luck,duck,suck(atleast one) ||gluck(more than one)
- It matches one or more characters; Similar to
?= Question Sign- It matches zero or one character; Similar to
.(dot), but it can skip matching - Eg:
lie?=lied(atmost one) ||lie(zero)
- It matches zero or one character; Similar to
|= Pipe Sign- It matches characters separated by
|pipe symbol. - Eg:
cat|dog= Matches eithercatordob; Similar to OR gate or operator.
- It matches characters separated by
()= Paranthesis- It match all characters that enclosed within it; Order of character matters.
- Eg:
(abc|def)= Matchabcordef; This is most commonly used for combining with other metacharacters when searching for texts.
3. Special Sequences
Special sequences are used to enhance the pattern matching by providing matching patterns logic and consist of \ (backslash) and alphabetical characters
Using backslash followed by a character, we can mention the pattern matching logic.
| Symbol | Meaning | Character Class / Description |
|---|---|---|
\b | Matches a word boundary (start or end of a word) | — |
\B | Matches not at a word boundary | — |
\w | Matches any word character | [a-zA-Z0-9_] |
\W | Matches any non-word character | [^a-zA-Z0-9_] |
\d | Matches any digit | [0-9] |
\D | Matches any non-digit | [^0-9] |
\s | Matches any whitespace character | [ \t\n\r\f\v] |
\S | Matches any non-whitespace character | [^ \t\n\r\f\v] |
\n | Matches a newline character | — |
\f | Matches a form-feed character | — |
\t | Matches a tab character | — |
\v | Matches a vertical tab character | — |
$ | Matches at the end of a string | — |
^ | Start of line/string | Start of any line (multi-line) |
$ | End of line/string | End of any line (multi-line) |
\A | Start of string | Only very beginning of string |
\Z | End of string | Only very end of string |
Notes:
- In regular expressions,
^and$are used for matching the start and end of a string, notAandZ. (as far as I know)- The backslash (
\) is required for escape sequences like\b,\d, etc.[a-zA-Z0-9_]is the character class for word characters (\w).[ \t\n\r\f\v]is the character class for whitespace (\s).
4. Character Classes
In simple terms, Character Classes helps finding set of characters easily. The charactes are enclosed within a square bracket[].
| Character Class | Description |
|---|---|
[abc] | Matches the letter a/b/c |
[abc][xz] | Match letter a/b/c followed by either x/z |
[^abc] | Match all letters except a/b/c. |
[0-9] | Match all digits from 0-9 (inclusive range) |
[a-z] | Match any lowercase letters |
[A-Z] | Match any uppercase letters |
[a-zA-Z] | Match lowercase/uppercase letters |
[a-zA-Z0-9_] | Match any alphanumeric character |
[m-p2-8] | Ranges between m-p letters and 2-8 numbers |
5. Occurrence Indicators
Some of the metacharacters are also considered as occurence indicators. It plays the vital role in determining the occurences of characters.
This is the basic occurence indicators:
| Occurrence Indicators | Description |
|---|---|
+ | Matches one or more |
? | Matches zero or one |
* | Matches zero or more |
These are some advanced that uses curly braces {}:
| Occurrence Indicators | Description |
|---|---|
{n} | Matches exactly n number of times |
{n,} | Matches at atleast n number of times |
{n,m} | Matches from n to m times |
{,n} | Matches upto n times |
References: