Regex startup
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Regular expressions (regex) are used to match patterns in text. They contain special characters called meta characters that represent expressions to match, like * for 0 or more matches and + for 1 or more matches. Regex can be used for text processing tasks like validating formats. The document discusses various regex meta characters, quantifiers, character sets, modifiers, grouping, backreferences, and lookahead/lookbehind operations. It provides examples of regex patterns for tasks like matching XML tags and validating email addresses.
![Meta Characters (Contd..)
Character Meaning
* 0 or more
+ 1 or more
? 0 or 1 (optional)
. All characters excluding new-line
^ Start of line. But [^abc] means
character other than 'a' or 'b' or 'c'
$ End of line
A Start of string
Z End of string](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-5-320.jpg)
![Meta Characters (Contd..)
Character Meaning
{ } If I know How many times the pattern
repeats I can use this
Ex: a{2, 5} matches 'a' repeated
minimum 2 times and maximum 5
times.
| Saying 'or' in patterns
Ex: cat|dog|mouse
() Used to capture groups
[ ] Only one letter from the set](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-6-320.jpg)
![Character Sets
Matches one character among the set of
characters
[abcd] is same as [a-d]
[a-di-l] is same as [abcdijkl]
[^abcd] matches any character other than
a,b,c,d
Quantifiers can be applied to the character sets
− [a-z]+ matches the string 'hello' in
'hello1234E'](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-10-320.jpg)
![Characters for Matching
Common character classes shorthand
[a-zA-Z0-9_] w
[0-9] d
[ tnr] s
[^a-zA-Z0-9_] W
[^0-9] D
[^ tnr] S
b Word Boundary
B Other than a Word Boundary](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-11-320.jpg)
![Simple Matching
modegunta.srikanth@gmail.com
− mail id should not start with number or special
symbols
− Mail id id can start with _
− Mail id can have '.' in the middle
− Should end with @domain.com
Pattern :
− [a-zA-Z_][a-zA-Z_.]+@w+.(com|co.in)
− Meta characters must be escaped in the
pattern to match them as normal characters](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-12-320.jpg)
![Grouping Example
If the command is
− /sbin/service <service-name> <command>
− ([^s]+)s+([w-_]+)s+(start|stop|status)
− Group 0=matched pattern
− Group 1=”/sbin/service”
− Group 2=<service-name>
− Group 3=<command>
− Command can be start, stop or status](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-15-320.jpg)
![Back references example
For example take the xml tag
− <root id=”E12”>test</root>
− <([w-_]+)s*([^<>]+)?>w+</1> matches
xml element
− Group 0: <root id=”E12”>test</root>
− Group 1: root
− Group 2: id=”E12”
− 1 in the regex pattern is the back reference to
group 1.](https://image.slidesharecdn.com/regex-startup-130829070056-phpapp02/85/Regex-startup-17-320.jpg)
Regex startup
- 2. Introduction Also referred to as Regex or RegExp Used to match the pattern of text − Ex: maven and maeven can be matched with regex “mae?ven” Regular Expressions are processed by a piece of software called “Regular Expressions Engine” Most of the languages support Regex − Ex: perl, java, c# etc.
- 3. Introduction (Contd..) Used where text processing is required. XML parsing involves Regex as it is based on the pattern matching. − We will see how to match xml or html tag. Automation of the tasks − Ex: if mail subject contains “<operation> <some task name> <command>” then start processing the task. Text Editors updating the comments to functions automatically(Replacing a pattern with some text) − Ex: replace − “sub subroutine(parameters){<statements>}” by /* this is a sample subroutine*/ sub subroutine(parameters){<statements>}
- 4. Meta Characters The following are the meta characters | ( ) [ { ^ $ * + ? .
- 5. Meta Characters (Contd..) Character Meaning * 0 or more + 1 or more ? 0 or 1 (optional) . All characters excluding new-line ^ Start of line. But [^abc] means character other than 'a' or 'b' or 'c' $ End of line A Start of string Z End of string
- 6. Meta Characters (Contd..) Character Meaning { } If I know How many times the pattern repeats I can use this Ex: a{2, 5} matches 'a' repeated minimum 2 times and maximum 5 times. | Saying 'or' in patterns Ex: cat|dog|mouse () Used to capture groups [ ] Only one letter from the set
- 7. Quantifiers To specify the quantity − Ex: ear, eaaaar – the quantity of a is 1 and 4 in these two cases. If a pattern is repeated then we need to use quantifiers to match that repeated pattern. To match the above case we use the following regex − ea+r means a can come 1 or more times
- 8. Quantifiers (Contd..) * 0 or more times (it is hungry matching) Ex: ca* matches c, ca, caa, caaa etc. Matches even if the character does not exist and matches any number of 'a' s generally till last occurrence of pattern + 1 or more times (it is hungry matching) Ex: ca+ matches ca, caa, caaa etc {n} Match exactly n times Ex: ca{4}r matches caaaar {m,} Matches minimum of m times and maximum of more than m times Ex: ca{2,}r matches only if a repeats greater than 2 times. (hungry matching) {m,n} Matches minimum m times and maximum n times. Ex: ca{2,3}r matches and 'a' repeats minimum 2 times and maximum 3 times. (hungry matching) Hungry Matching refers to the behavior that the pattern matches maximum possible text. Ex: for ca{0,4} the text “caaaa” matches I.e all the 4 'a's are matched.
- 9. Quantifiers (Contd..) *? Lazy matching i.e it matches 0 or more times but stops at first match Ex: if text is “caaaaaa” then “ca*?” will match only 'c'. +? Lazy matching i.e it matches 1 or more times but stops at first match Ex: if text is “caaaaaa” then “ca+?” will match only 'ca'. ?? Lazy matching i.e it matches 0 or 1 times but stops at first match Ex: if text is “ca” then “ca??” will match only 'c'. {min,}? {n}? {min, max}? Lazy matching Lazy Matching refers to the behavior that the pattern matches minimum possible text. Ex: for ca{0,4}? the text “caaaa” matches only “c”
- 10. Character Sets Matches one character among the set of characters [abcd] is same as [a-d] [a-di-l] is same as [abcdijkl] [^abcd] matches any character other than a,b,c,d Quantifiers can be applied to the character sets − [a-z]+ matches the string 'hello' in 'hello1234E'
- 11. Characters for Matching Common character classes shorthand [a-zA-Z0-9_] w [0-9] d [ tnr] s [^a-zA-Z0-9_] W [^0-9] D [^ tnr] S b Word Boundary B Other than a Word Boundary
- 12. Simple Matching modegunta.srikanth@gmail.com − mail id should not start with number or special symbols − Mail id id can start with _ − Mail id can have '.' in the middle − Should end with @domain.com Pattern : − [a-zA-Z_][a-zA-Z_.]+@w+.(com|co.in) − Meta characters must be escaped in the pattern to match them as normal characters
- 13. Modifiers Modifier Meaning i Case insensitive g Global matching (in perl) m Multiline matching s Dot all ('.' matches n also) x Extended regex pattern (pretty format ref: perl) e (Used for replacing string) evaluate the replacing pattern as an expression (ref: perl)
- 14. Grouping Groups can be captured using parenthesis − (<pattern>) − Saves the text identified by the group into a backreference (we will see it later) Groups are to capture part of text in the matching pattern − Ex: take simple xml element <root>test</root> − <(w+)>.*?</1> − Here 1 is back reference Java has a method “group(int)” method in “java.util.regex.Matcher” class.
- 15. Grouping Example If the command is − /sbin/service <service-name> <command> − ([^s]+)s+([w-_]+)s+(start|stop|status) − Group 0=matched pattern − Group 1=”/sbin/service” − Group 2=<service-name> − Group 3=<command> − Command can be start, stop or status
- 16. Back References Stores the part of the string matched by the part of the regular expression inside the parentheses If there is any string that occurs multiple times in the input, we can use back reference to identify the match Ex: xml/html start-tag should have the end-tag Here if we capture the start-tag name in first group, we can put end-tag name as back reference (1)
- 17. Back references example For example take the xml tag − <root id=”E12”>test</root> − <([w-_]+)s*([^<>]+)?>w+</1> matches xml element − Group 0: <root id=”E12”>test</root> − Group 1: root − Group 2: id=”E12” − 1 in the regex pattern is the back reference to group 1.
- 18. No grouping with parenthesis If groups are not required for the parenthesized patterns − Use ?: inside group (?:) − (text1|text2|text3) is any on of text1, text2 and text3 − (?:text1|text2|text3) but will not be a group
- 19. Look ahead and Look behind Positive look-ahead − w+(?=:) not all words.... select words that come before ':' Negative look-ahead − w+(?!:) words other than those coming before : When the pattern comes the regex engine looks ahead for the filtering pattern in case of Look ahead. Positive look-behind − (?<=a)b selects 'b' that follows 'a' Negative look-behind − (?<!a)b selects 'b' that doesn't follow 'a' When the pattern comes the regex engine looks behind for the filtering pattern in case of Look behind.
- 20. References: 1) http://www.regular-expressions.info/tutorial.html 2) Thinking in java 4th Editon – Chapter: Strings page 392
- 21. Thank You