Regular Expressions and Pattern Matching

Transcription

1 Regular Expressions and Pattern Matching Regular Expression (regex): a separate language, allowing the construction of patterns. used in most programming languages. very powerful in Perl. Pattern Match: using regex to search data and look for a match. Overview: how to create regular expressions how to use them to match and extract data biological context

2 Parse files of data and information: fasta embl / genbank format html (web-pages) user input to programs So Why Regex? Check format Find illegal characters (validation) Search for sequences motifs

3 Simple Patterns place regex between pair of forward slashes (/ /). try: #!/usr/bin/perl while (<STDIN>) { if (/abc/) { print 1 >> $_ ; Run the script. Type in something that contains abc: abcfoobar Type in something that doesn't: fgh cba foobar ab c foobar print statement is returned if abc is matched within the typed input.

4 Can also match strings from files. Simple Patterns (2) genomes_desc.txt contains a few text lines containing information about three genomes. try: #!/usr/bin/perl open IN, <genomes_desc.txt ; while (<IN>) { if (/elegans/) { #match lines with this regex print; #print lines with match Parses each line in turn. Looks for elegans anywhere in line $_

5 Flexible matching There are many characters with special meanings metacharacters. star (*) matches any number of instances /ab*c/ => 'a' followed by zero or more 'b' followed by 'c' => abc or abbbbbbbc or ac plus (+) matches at least one instance /ab+c/ => 'a' followed by one or more 'b' followed by 'c' => abc or abbc or abbbbbbbbbbbbbbc NOT ac question mark (?) matches zero or one instance /ab?c/ => 'a' followed by 0 or 1 'b' followed by 'c' => abc or ac

6 More General Quantifiers Match a character a specific number or range of instances {x will match x number of instances. /ab{3c/ => abbbc {x,y will match between x and y instances. /a{2,4bc/ => aabc or aaabc or aaaabc {x, will match x+ instances. /abc{3,/ => abccc or abccccccccc or abcccccccccccccccccccccccccccccccccccccccccccccc cccccccccccccccccccccccccccccccccccccccccccccccc ccccccccccccccccccccccc

7 More metacharacters dot (.) refers to any character even tab (\t) and space but not newline (\n). /a.*c/ => 'a' followed by any number of any characters followed by 'c'

8 Escaping But I want to use these symbols in my regex!?! to use a *, +,? or. in the pattern when not a metacharacter, need to 'escape' them with a backslash. /C\. elegans/ => C. elegans only /C. elegans/ => Ca, Cb, C3, C>, C., etc... The 'delimitor' of the regex, forward slash /, and the 'escape' character, backslash \, are also metacharacters. These need to be escaped if required in regex. Important when trying to match URLs and addresses. /joe\.bloggs\@darwin\.co\.uk/ /www\.envgen\.nox\.ac\.uk\/biolinux\.html/

9 Using metacharacters. The file nemaglobins.embl contains 21 embl database files that contain a globin protein within their sequence. try: #!/usr/bin/perl $count; open IN, <nemaglobins.embl or die; while (<IN>) { if (/AC.*/) { #that's three spaces print; $count++; print total=$count\n ;

10 Grouping Patterns Can group patterns in parentheses (). Useful when coupled with quantifiers /elegans+/ => eleganssssssssssssss /(elegans)+/ => eleganselegans...elegans 1 2 n /eleg(ans){4/ => elegansansansans

11 Alternatives Want either this pattern or that pattern. Two ways: 1.) the vertical bar ' ' either the left side matches or the right side matches /(human mouse rat)/ => any string with human or mouse or rat. Combine with previous examples: /Fugu( \t)+rubripes/ matches if Fugu and rubripes are seperated by any mixture of spaces and tabs

12 2.) character class is a list of characters within '[]'. It will match any single character within the class. /[wxyz1234\t]/ => any of the nine. a range can be specified with '-' /[w-z1-4\t]/ => as above to match a hyphen it must be first in the class /[-a-za-z]/ => any letter character and a hyphen negating a character with '^' /[^z]/ => any character except z /[^abc]/ => any character except a or b or c

13 Other Shortcuts \d => any digit [0-9] \w => any word character [A-Za-z0-9_] \s => any white space [\t\n\r\f ] \D => any character except a digit [^\d] \W => any character except a word character [^\w] \S => any character except a white space [^\s] Can use any of these in conjunction with quantifiers, /\s*/ => any amount of white space

14 Using alternatives to find a hydrophobic region... try: open IN, "< nippo_sigpept.fsa" or die; while (<IN>) { if (/>/) { #a header line $count++; #keep running total of sequence number else { #not a header if (/[VILMFWCA]{8,/) { $match++; print "Hydrophobic region found in $match sequences from $count\n"; Could also have used /(V I L M F W C A){8,/

15 Revisited? So far matching against $_ Binding Operator The binding operator =~ matches the pattern on right against the string on left. Usually add the m operator (optional). $sumthing = 'Ascaris suum is a nematode'; if ($sumthing=~m/suum.*nematode/) { print this organism infects pigs!\n ;

16 Anchors /pattern/ will match anywhere in the string. Use anchors to hold pattern to a point in the string. caret ^ (shift 6) marks the beginning of string while dollar $ marks end of a string. /^elegans/ => elegans only at start of string. Not C. elegans. /Canis$/ => Canis only at end of string. Not Canis lupus. /^\s*$/ => a blank line. $ ignores new line character \n. N.B. compare use of ^ as an anchor with that in the character class.

17 Word Boundary Anchors (2) \b matches the start or end of a word. /\bmus\b/ would match mus but not musculus /la\b/ => Drosophila but not Plasmodium /\btes/ => Comamonas testosteroni but not Pan troglodytes \b ignores newline character. Be careful with full stops they're characters too!

18 Memory Variables Able to extract sections of the pattern match and store in a variable. Anything stored in parentheses () is written into a special variable. The first instance is $1, the second $2, the fourth $4 and so on. Extract from file: Organism: Homo sapiens... Extract from Perl script: while ($line=<in>) { if ($line=~m/organism:\s(\w)+\s(\w)+/) { $genus=$1; #stores Homo $species=$2; #stores sapiens

19 Substitutions Able to replace a pattern within a string with another string. Use the s operator s/abc/xyz/ => find abc and replace with xyz By default only the first instance of a match. Using 'g' modifier (global) will find and replace all instances. $line = 'abccdcbabc'; $line =~ s/abc/xyz/g; print $line; #produces xyzcdcbxyz; Run dna2rna.pl Now look at dna2rna.pl 1 2

20 dna2rna.pl #!/usr/bin/perl print "Enter DNA sequence\n"; while ($line = <STDIN>) { chomp $line; #remove trailing \n if ($line=~m/[^agct]/i){ #case insensitive infered by 'i' #modifier print "your sequence contained an invalid nucleotide: $&\nplease try again\n"; #'$&' is a special variable which stores what the #regular expression matched. Don't worry about it for now. else { $line=~s/t/u/g; #replace all lower case 't' $line=~s/t/u/g; #replace all upper case 'T' print "The RNA sequence is:\n$line\n"; print Try again or ctrl C to quit\n ;

21 EMBL file revisited using shortcuts and anchors to help make more robust: if (/AC.*/) { #that's three spaces can be rewritten as; if (/^AC\s{3(.*)\n$/){ #more certain to return what you want $accession=$1; #now have info stored to use later.

22 Now Its Your Turn :o) nemaglobins.embl contains entries for complete cds of nematode sequences. Foreach entry print the ACcession, OrganiSm name and AGCT content of the SeQuence. Output should read: Accession: AC00000 <tab> Species: Toxocara canis <newline> A: 34 G: 65 C: 24 T: 75 <newline><newline> Hints: The lines of interest are AC, OS, and SQ. Three regular expressions - one for each query. Use a series of if and elsif loops to search for regular expressions. Print when matched. Bonus point - remove the semi-colon from the accession id. Shout if need help.