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< h2 id = "backreferencing" > Backreferencing< / h2 >
< p > Backreferencing is used to match same text again. Backreferences match the same text as previously matched by a capturing group. Let's look at an example:< / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vuap" class = "embed" > < / iframe >
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< p > < strong > Note:< / strong > < code > \/< / code > is escaped < code > /< / code > character, check it out in the appendix.< / p >
< p > The first captured group is (< code > \w+< / code > ), now we can use this group again by using a backreference (< code > \1< / code > ) at the closing tag, which matches the same text as in captured group < code > \w+< / code > .< / p >
< p > You can backreference any captured group by using < code > \group_no< / code > .< / p >
< p > Let's have two more examples:< / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vuas" class = "embed" > < / iframe >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vube" class = "embed" > < / iframe >
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< h3 id = "backreferencingandcharacterclass" > Backreferencing and character class< / h3 >
< p > Backreferencing can not be used in character class. Let's see an example:< / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vu7p" class = "embed" > < / iframe >
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< h3 id = "backreferencingandquantifiers" > Backreferencing and quantifiers< / h3 >
< p > When we are using a backreference for an expression with quantifiers, then we have to be careful. Let's observe it:< / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vu85" class = "embed" > < / iframe >
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< p > Note that < code > (\d)+< / code > and < code > (\d+)< / code > both are different. So, what will happen for < code > (\d)+ -- \1< / code > expression and same text above?< / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vu85" class = "embed" > < / iframe >
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< p > Can you observe something?< / p >
< p > For < code > (\d)+ -- \1< / code > expression and < code > 123 -- 3< / code > string, first time 1 was stored in \1, then 2 was stored in \1 and at last 3 was stored. So, it will show a match if and only if the last character before < code > --< / code > is exactly same as the character after < code > --< / code > .< / p >
< p > < strong > Problems:< / strong > < / p >
< ol >
< li > < p > Match any palindrome string of length 6, having only lowercase letters.
Answer: < code > ([a-z])([a-z])([a-z])\3\2\1< / code > < / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vubk" class = "embed" > < / iframe >
< / div > < / li >
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< li > < p > < strong > RegEx< / strong > : < code > (\w+)oo\1le< / code > < br >
< strong > Text:< / strong > < code > google, doodle jump, ggooggle, ssoosle< / code > < / p >
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< p > Answer: < / p >
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< iframe scrolling = "no" style = "position: absolute; top: -9999em; visibility: hidden;" onload = "this.style.position='static'; this.style.visibility='visible';" src = "https://regexr.com/4vubk" class = "embed" > < / iframe >
< / div > < / li >
< / ol >
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< p > < strong > Note:< / strong > For group numbers more than 9, there is a syntax difference.< / p >
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