grep (bash) multi-line pattern

Question

In bash (4.3.46(1)) I have some multi-line so called fasta records where each record is initiated by on line with >name and the following lines DNA sequence ([AGCTNacgtn]),

Answer 1

Created sedgrep mixed version to support in generic way... You could use this sedgrep shell command available at https://github.com/iamdvr/sedgrep-shell-util

Direct Link: https://github.com/iamdvr/sedgrep-shell-util/blob/main/sedgrep

For your case direct command is this...

cat  | sed -nr ':main; /^>.*chr2/ { :loop; p; n; /^>/ b main; b loop} '

sedgrep usage is as follows...

Default NEW_LINE_PATTERN is ^\[
Usage : 
    cat {INPUT_FILE_NAME}  | sedgrep  {NEW_LINE_PATTERN} {THREAD_OR_SEARCH_PATTERN} 
    cat {INPUT_FILE_NAME}  | sedgrep  {THREAD_OR_SEARCH_PATTERN} 
    sedgrep {NEW_LINE_PATTERN} {THREAD_OR_SEARCH_PATTERN} {INPUT_FILE_NAME}
    sedgrep {THREAD_OR_SEARCH_PATTERN} {INPUT_FILE_NAME}
Example : 
    cat sampleInput.log | sedgrep 2016-05-23 DB_CONN
    cat sampleInput.log | sedgrep DB_CONN
    sedgrep 2016-05-23 DB_CONN sampleInput.log
    sedgrep DB_CONN sampleInput.log